You say they are identical, but are they really? There are lots of things that might be different between the modules but don't necessarily go into the published specifictions. One of those things is density. The modules can contain the exact amount of DRAM but use different chips that have different densities and therefore a different count of chips on the module and that can keep them from working together. That's an extyreme example but ther are others.
It's best to stick with kits. Otherwise you can throw the dice and get two separate modules. Maybe they'll work and maybe not and maybe for a while and then fail later.