Maybe it would help if you explained why you want to do this as it seems a bit odd? As Nick says, if one side has actually worn out quicker than the other you need to find out why as it means there's something important and possibly dangerously wrong.
The only issue I can think of is that even if you used the same make of pad as was in the other side, you are going to have, initially at least, an imbalance in braking from one side to the other. This would be a big problem in a car as it could send you off the road if you tried to stop in a hurry. On a bike I can't see it being any different to just having a single disk, but it will produce a twisting action in the forks which isn't ideal.
Bottom line is you should endeavour to replace both sets at the same time and dont mix makes of pad.