But if you do encounter issues folk will remind you, you mixed. Since the machines are not standardized and changes are so many, then you can fix a cranky PC with mixed RAM (sometimes) by moving sticks around because the BIOS used to read the SPID chip for settings from only the first RAM it encountered, this means that the problems can't be dismissed even in your scenario.
That all aside you can mix. But no tech I know will tell any one to mix and not worry. So why not give it a shot?
I want to add more RAM to my system. I currently have Corsair Vengeance Pro DDR3 2133MHz 4GB x 2 (CMY8GX3M2A2133C11). Is it possible to mix it with exactly the same RAM, just 2 x 8GB (CMY16GX3M2A2133C11), to create 24GB system?