It kind of depends on what kind communication you're looking for.
JMS is a fine choice. It's portable and not a total bear to set up (at least in GF, it's different in something like Tomcat which doesn't provide JMS).
It's also good if you using it to broadcast notifications or even queries from one webapp to the other.
But if you're looking for something like shared state between the two webapps, then you might want to look at some of the caching libraries, and putting those libraries higher up the classloader chain (specifically outside of your webapps) so they can actually share the implementation.
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