No matter the brand, speed, and size of the RAM,theoretically,you can mix and match RAM in your system, though it may prove to be problematic. Also, depending on the setup & workload, you may not even notice a difference in performance (Read:When you can’t make use of more RAM)....
You might be able to run those together if there's a driver that supports both, which I'm pretty sure there are But whether it'll stop the stuttering depends on what's being loaded on when you're running your things And you'll need a power supply sufficient enough to drive both GPU...