It depends on on the motherboard. Some motherboards (especially older ones) require that you have two of the same chips in order to be recognized. Additionally, it may simply be that the 64 MB chip is bad or improperly inserted which is why it doesn't show up. Check the documentation on your motherboard to find out the max amount of RAM it can support, and take out and reinsert the 64 MB chip to ensure it's properly seated.

AJ