Simple answer:
No problem.

Elaborate answer:
The RAMs can have different clock but the mobo will down-clocked the faster RAM to follow the slower RAM speed.

More technical answer:
Mixing RAM might not work properly if you have RAM with different timing. Those RAM should have same CAS latency, timings and recommended voltage. However, this is not guarantte that you will experience BSOD (blue Screen of Death). The RAM might hate each other or your mobo might prefer the other RAM.

So, the safer bet is to have both RAM from same maker as well.