Can I use two 16GB RAM sticks if my motherboard says "16GB max" per slot?

Probably yes — if the “16GB max” you saw is per slot, not total. Most motherboards list max per slot and max total separately. If it’s 16GB per slot and you have two slots, two 16GB sticks will work fine. Just check the manual for “Max Memory Capacity” to be sure, and make sure you’re running a 64-bit OS.