Firstly you need to check the specification of your computer motherboard. Actually you can install 2+1 gb ram but sometimes error occurred due different memory size. But you should install same frequency of ram properly on both slot.
That is something that i have come across in my computer, my mobo manual says i can have only 8 GB of ram-- I don't have the money or the time to buy more ram and find out for you but I don't really see the problem, install it and see if it boots, if it does see if the mobo can use all 2GB. if the mobo says only 1 GB per slot, it might just use 1 GB out of the 2 GB module you have. Try it and let us know!