As Brian says, Don't Do It! Epoxy requires a complete chemical reaction to harden completely. While the basic chemistry all starts with the material made by one or two manufacturers, the amounts of stuff that ends up in each blend is different. You can not be sure that there will be sufficient constituents in different mixes to allow for full and complete chemical mixing.

If you were to mix up a appropriate ratio mix of one blend and another correct ratio of some other manufacturers blend and then mix those two batches together, I would say you are fine. But resin from one and hardener from another is a bad idea.

You can make a test batch and see what happens, but even if you get what seems like a fully hardened mix, you can't be sure of what will happen in a years time.