The question isn't as simple as all that. In fact, neither is the answer.
The odd man out, (Or, mixed memory)
While memory modules that did not come together in a matched set that was tested by the manufacturer to be compatible, certainly CAN still work together, often it does not. Right up front I'll tell you that if you are trying to get sticks to work in the same machine together that were purchased separately, even if they are otherwise identical according to the kit or model number or if they would seem to have identical timings and voltage requirements, there is a very good chance that you simply will not be able to do that. There is also a pretty fair chance that you might be able to if you are willing to take your time, listen to and understand what you are being told and follow the steps necessary to determining if they will "play nice" or not.
The exception in most cases will be that if the memory from both sets are the same speed and timings and both kits are within the JEDEC specifications for the default speed on that platform, so for example, 2666mhz on the latest Intel Z390 platform, 2133mhz on Ryzen first and second Gen platforms, then they stand a much better chance of working together but if they are higher speed kits the chances begin to diminish from what they might be at the low speed and loose timings end of the scale.
A word of advice. If you just purchased this memory, and for whatever reason you bought two separate sticks of the same memory instead of buying them together in a matched set, see if you can return them for a refund or credit towards buying a similar or same set of matched sticks that come together in a kit. It is ALWAYS better to have matched modules because from brand to brand, or even within the same brand, in fact, even when the part numbers are IDENTICAL, there can be anything from simply slightly different memory chips that were sourced from different bins at the end or beginning of a production run to entirely different configurations altogether even though the model numbers seem to be the same. Some manufacturers even reuse model numbers when they discontinue a product. Point being, memory is only the same for sure when all sticks came out of the same blister pack or packaging and were sold as a tested kit.
In order to determine if differences in the memory, or a need for increased voltage when using more than one stick (Especially if you are running three or more sticks) are responsible for the problems you are having you will always want to begin your troubleshooting process by attempting to boot the machine with only a single stick of memory installed. Also, for practically every consumer motherboard that's been sold since at least as far back as about 2014, the A2 memory slot which is the second slot over from the CPU socket, is THE slot that is most commonly designated for the installation of a single memory module. Slots A2 and B2 are almost always the slots specified in the motherboard memory population rules for use with two modules. If you need to install a third module I have no opinion on which of the remaining slots to use for that, but typically since the A1 slot is right next to the CPU socket and often interferes with the CPU cooler or fan, I'd say the B1 slot was probably just as good.
Honestly, I don't ever recommend that you HAVE three modules installed anyhow. Using memory in pairs is almost always a better option, except on boards that support triple channel memory population, so that normal dual channel operation will occur. And that's another thing. When it comes to memory there are no "single channel" or "dual channel" memory modules. There are ONLY memory modules and the motherboard and CPU architecture will determine whether or not dual, triple or quad channel operation is possible based on the architecture and how many modules are in use. Occasionally though there are situations where it might make sense to run three modules and some boards CAN use three modules in a FLEX type mode where two of the modules will operate in dual channel while the third oddball module will run in single channel. I'd avoid oddball configurations though if possible because many motherboards will simply run ALL modules in single channel mode when an odd number of modules are installed.
The fact is that most of what determines how well (Or not) two disparate memory modules are going to work together often has very little to do with those specifications at all in reality. It is really down to the more subtle differences of:
What speed are the memory modules? Because with lower speed memory kits (Depending on the platform of course, there are always additional factors and considerations), which for MOST DDR4 platforms we will assume to be within the JEDEC strictures of 2666mhz or lower, it seems there is a lot more forgiveness from the majority of platforms when mixing memory than with higher speed kits. Once you go past 2666mhz, and I'm talking even with one stick that is higher speed, outside the JEDEC specifications, the chances you might have a problem go up considerably. It can of course still work, but it can just as well not work at all or have some combination of mystery issues that don't even seem like they would be memory related but are.
What is the actual makeup/configuration of the memory modules themselves? What IC's (memory chips) were used? Because there is clearly something to be said for the fact that putting two sticks of RAM together that use ICs from entirely different manufacturers may tend to have less probability of working together than sticks using the same brand or type of ICs. And again, in other cases it might not matter at all and will still work. Also HOW the module is built. Is it a single or dual rank module? Is it a dual or single row module? What size of ICs are used, because some motherboards might not be able to compensate and find settings that work for both modules if there are broad differences in what is acceptable and can enable POST for one stick as compared to another, and if that's the case then probably no amount of manual configuration is going to help in that scenario. And these things are something you are very unlikely to ever find included on any list of specifications. You'd have to find a model specific review of that memory kit, which is usually pretty unlikely since memory kits don't tend to get much love in terms of reviews since there is very little to differentiate between them all aside from the very boring details under the hood that 90% of people care nothing about because they ARE going to only install matched sets and it will never be a concern for them.
It's sort of like putting 85 octane gas in your car when the owners manual specifically states that you should use 93. Sure, it WILL run on it, and you MIGHT not have any problems at all. There are however many vehicles out there that if you run 85 in them will experience anything from minor symptoms such as poorer gas mileage to severe issues like preignition/predetonation along with the knock it causes, which is bad. Obviously, not an exact analogy, but similar in that you could get anything from no problems at all to severe problems, and there are additional factors that could amplify or reduce any issues such as what elevation you are at, much as with mixed memory the motherboard in question might also be a factor since the higher the quality of the motherboard the better chance you have of small differences not becoming a problem.
TLDR; it depends on a lot of different factors, not all of which are probably going to be understood by the average person or even myself, considering I am no engineer. At all.
Although I helped my cousin upgrade from 8gb to 24gb on his fx based system and it worked great.
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