This is exactly the problem that PHP namespaces are designed to solve. There is no clean way to do this until 5.3. The best you can do is rename one or both and go through libraries that fix the calls. This is a pain, but since they are separate libraries, it should not be too complicated.
My other suggestion is to make sure that installing 5.3 is really impossible. This is by far the best solution to your problem.
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