This^.
Mixing different DIMMs from different packs is not a good idea.
One of three things will happen:
1, They will work together.
2, They work together but with issues of lower latency and/or clock speeds, instability or crashing.
3. They won't work at all.
Ideally you should buy a 2 x kit of whatever capacity. e.g 2 x 32gb.
What are your PC specs? If you are running a Ryzen system, maybe Zen 3 for example, then 2x 32 3600MT/s would be better.
And if you have an iGPU (not a discrete GPU) then faster ram really helps.