mileage may vary when using different RAM that didn't come together in the same kit.
Having varying capacities can further increase the chance of complications.
Even further more is if not all sticks have the exact same speed, timings, and voltage.
If your mobo has 4 slots and not more, then this means you plan to populate all the slots. Best bet is to make sure at the least that each kit runs in dual channel.
Refer to your motherboard manual for dual channel configuration.
Usually though, it goes like this:
Out of the four slots, the first two are going to be channel A respectfully A1 and A2
The second two slots next to those will be channel B respectfully B1 and B2.
When running dual channel, you want the two sticks that will be running together to be in identically numbered slots for A and B.
for example, you have a kit of 8GB that is comprised of two 4GB sticks.
One of those will go into A1 and the other will go into B1.
With your second two stick kit you will put one of them into A2 and the other into B2.
Again, refer to your motherboard manual for these slot placements as sometimes manufacturers don't put them in a row like A1 A2 B1 B2 but will instead group the dual channels together so it goes A1 B1 A2 B2. this second configuration option is kind of rare and not likely but good to check anyways.
If you are lucky enough, it's possible the manufacturer labeled the slots right on the motherboard itself and you can just check that.
But there is no guarantee for compatibility when using different sticks that didn't come in the same kit together so it ends up being a try it and see if it works sort of thing.
If it doesn't, you could either try and return the sticks, or sell the 8GB kit and keep the 16GB kit as your upgrade.
Or not sell the 8GB kit and keep them in a safe place as backups in case something happens to the 16GB kit.