Wirelessly posted (BlackBerry8703e/4.1.0 Profile/MIDP-2.0 Configuration/CLDC-1.1 VendorID/104)

To answer the OP's question, yes: you can do exactly what you propose. Both AirTrain routes have the same boarding platform. The airport-only route usually operates along the inner track, and the route that goes off airport usually operates on the outer track. Electronic signs on the platform indicate the destination of each arriving train.

That said, I agree with jbcarioca, and would consider doing what the OP proposes only if the AirTrain to Terminal 8 arrived before the AirTrain to Howard Beach arrived.