The laptop may not allow you to mix 12800 with 8500 at all.
However, if it does work you would have to figure out how much you really need the extra 2 GBs. Most people probably wouldn't notice the gain of 2. Most people probably also wouldn't notice the loss of speed from both dropping to 8500 either.
I would just stick with what is already in there. Either that or get a second copy of the Hynix one.