One thing I caught.
"I have two TF3D48G2666HC15B01 "
Part numbers are for a single stick of memory @ 8GB.
Mixing ram is not always stable.
For dual channel you need2 matched sticks from a set. not 2 single sticks.
Memory has a ton of secondary timings that must also match for stability.
That is why they are sold as Kits. The modules are tested to by the manufacturer to work together.
Buying 2 single sticks and putting them together is a crap shot if they work together.
Yes, I know, but on the day I went to buy there was no kit available, only individual ones. I've been using it for 1 year, apparently without problems and with all the timings identical. In a near future I will upgrade to 32GB, but I will consider only kits, maybe something like 32GB (2x16) 3200 CL16.Yes, good catch. Something I usually latch onto right away, and I agree, if those sticks did not come in a kit TOGETHER, then you're fortunate they are even running together, much less attempting to overclock them. I'd recommend that you run them at the stock XMP profile speed rather than overclocking them, if they were not both a part of a matched set. And these days, to be honest, it doesn't make much sense not to simply just BUY DIMMs that are the speed you want to run.
You don't actually NEED "two matched DIMMs from a set" in order to run in dual channel. Being matched has nothing to do with running in dual channel, but what DOES matter, is that the stick are compatible ENOUGH to run together in dual channel, and by that we mean that the ENTIRE configuration of the DIMM (ICs, ranks, rows, primary, secondary and tertiary timings) are similar enough that the motherboard is able to make micro adjustments that are amenable to both sticks. Sometimes both sticks look at the motherboard and say "no, thanks, we're not playing that game", while other times they are more like "ok, I guess we'll play", and still others when the memory is very much the same or is matched simply works together with no micro adjustments to any of the timings being necessary by the motherboard.
Besides which, dual channel operation isn't the issue in question here really AND usually, if the DIMMs are specc'ed within the JEDEC standards for the platform (So, for Z490, 2666mhz or less) there is usually a LOT more forgiveness for differences between DIMMs and not using matched ones. Outside of the JEDEC standard configurations, and anytime the XMP "OC" or an actual manual OC is involved, you quickly begin to move further and further away from that ESPECIALLY when you start trying to tighten timings on top of it. My further advice on this would be, if you're going to do something other than simply run them at their XMP profile values, then either overclock them OR tighten the timings, but not both. Especially not when (Or IF) they are not from a matched set.
About my CPU OC thing, i'm starting to get extremely confused, because I can use 5100 mhz through XTU, with a very low Vcore. But if I replicate this result in the BIOS, my PC just crash.