I'm trying to figure out what you mean by "double-sided." The best I can figure is that you mean: both sticks are in the two slots (of the four total slots) that are next to eachother and you want to know if you can put a single 1 gig stick in one of the remaining two slots.
The short answer is: yes.
The longer answer: If you have DDR2 as opposed to DDR memory, then you want the two 512MB sticks in slots 1 and 3 or 2 and 4 (if slots look like this: || || --> 12 34) in order to run dual channel. then you can put the 1 gig stick in an empty slot. If you don't have DDR2 memory, then leave them where they are and put the 1 gig stick in an empty slot.