Mixing Two MP3s in GarageBand 09
What you're asking could be done but chances of failure are too high. The reason is you could use a plugin to alter the tempo of one of the MP3s but that will introduce two big problems:


If you change it beyond certain threshold, pops and clicks will start to be heard.
It will be really hard to sync them. It may look in tempo at the beginning, but a very small tempo difference will be noticeable as the tracks advance, which will require lots of micro adjustments (not good). 


My advice is to take the tracks to an Apple Store and ask the guys there if you can use a Garageband for a couple of hours. They will rarely have any problem with that. There you can use Garageband or even a Logic Express (or pro!), that have a better Flex feature.