I don't understand the question. Can you use computer A while you're using computer B? Are you talking about network rendering? If so, then consider that both computers must be able to run After Effects and open the project that you're working with. Because After Effects CS4 only runs on processors with the Intel architecture (not PowerPC chips), the "non-Intel" Mac is a non-starter.
Yes, i was looking for ways to speed up rendering and I have an old mac collecting dust under my desk. Thought I would see if there is any way I can utilize the machine's memory to speed up my machine outside of buying more memory for my newer machine which I may have to do.