What platform do you have, X58?
You could combine them if you wanted, ideally they'd be rated for the same speed, timings and voltage. Just make sure you have 1x 4GB and 1x 2GB in each channel so they're all balanced.
If it's the board you linked in the other thread you both wouldn't have room for all of the sticks and you would have an imbalance by using 3 sticks instead of 2 or 4.