This is dangerous and I've never seen it done correctly. It's possible, but you attempt it at your own risk.
You may need to change the configuration depending on your container. Cross-context communication is typically disabled by default as it could pose a security risk if one application is malicious.
You can
reach from context to another using getServletContext().getContext(contextName), you can then get the request dispatcher and forward as normal. Whether or not it works is a matter of chance
One problem you may encounter is that crossing from one context to another may re-enter through the
thread pool, so if you are doing an include you have no control over the ordering of the two included resources! There's more to this but it is too vendor specific to warrant discussion. As I said, dangerous and hard to get right.