Er... I'm not sure if I understand what you are asking.
Lets say I have two images, both seperate images:

AND

Both 50x50 Pixels. I would like to make them into ONE 100*50 Pixel image --

However, I would like to do this on the fly, as the actual images I would be using are dynamically generated.
There IS a reason I'm trying to do it this way. 😛