Three possibilities (I think): 1 - It works, but MB will adjust itself to accommodate the slower CAS 10. 2 - It won't work, but the BIOS will tell that at power on. 3 - It appears to work, but when you check the Properties on My Computer, only half the memory appears.
There's a 4th. It might not work at all, in which case you'll get boot failure. Best bet is to check the manual and see if the DRAM controller will handle the different speeds gracefully.
I think newer motherboards care less about CAS mixing. I haven't really had any issues I can think of when it comes to mixing RAM. I say go for it. Only one way to find out whether it works or not. I have a P6T (1366) with 4 different brands and sizes of RAM in it. Never had a problem running 11GB. I've used it for data recovery for years. Recently replaced it with an AMD Gigabyte board because it has USB 3.0.