If both memories are compatible, it will work. However such configuration is not advisable. In order to have memory working in Double Channel Mode (i.e. double the memory frequency band), two same P/N modules should be installed.
Mixing memory sizes is generally something you should not do, which is why memory is sold in kits. You certainly would not gain any performance from the mixed memory (i.e. dual channel).
Now, that said, I have seen customers and configurations where they do mix memory sizes. Whether it works or not is dependent on several things.
So, the best I can say is to try it, but I do not recommend it.