Ram will run at the slowest speed. So if you mix a 3200 with a 2400, both will run at 2400.
To be honest, mixing ram that came from 2 different kits is never a good idea. Even if the ram is exactly identical vendor, model, speed, color etc. You only see the primary timings, the 15-16-15-39 etc, what you don't see is the 40+ secondary and tertiary timings. All it takes is one to be slightly off, and the whole thing either requires voltage bumps, loosening timings even dropping speeds to get both sticks to play nice. Sometimes even all that doesn't work and both won't work at all together. So it's always advised to buy all the ram in a single kit, a single package, not seperates, as ram is tested by the factory and have certified the ram works.
If you want 2x8Gb, you should sell the 1x8Gb you own, and buy a 2x8Gb kit in 2400/2666 or whatever you like.