Mixing shouldn't be an issue, just make sure they're both the same speed. However, you'll be limited to how many sticks you can add, and may not be able to utilize all of your available slots once the ranks add up. It's also entirely dependent of the system board. How many memory slots do you have? My workstation has 8 slots, altogether. In this case, you could either have a maximum of 8 of the 1Rx8 sticks or 4 of the 2Rx8 sticks. 8 ranks would be the maximum supported. Therefore, a single rank (1Rx8) would count as one, whereas a dual rank (2Rx8) would count as two, all of which would be added up to a total maximum of eight (8 times 1R or 4 times 2R, or any combination, thereof, that would total "eight").