Mixed RAM modules are not guaranteed compatibility, RAM is only guaranteed in the form sold (from the same packs) - regardless as to the make/model or capacity of the board.
Not saying it never works, it's just a 50/50 bet as 2 modules of the exact same model can be manufactured differently and also be comprised of different materials and parts.
if they are the same timings and voltages, you won't damage anything by trying and seeing. But if the RAM has different power requirements, then it is not recommended.