You can setup a reverse proxy on Apache A, so that all requests for your app go to Apache B with a config like:
ProxyPass /myapp http://apacheb.domain.com/myapp
ProxyPassReverse /myapp http://apacheb.domiain.com/myapp
Then, using ModJK on Apache B, you set up a workers.properties file to define your tomcat servers and any load-balancing and connection parameters. In your vhost block, you set JKmounts to the tomcat server. So, if you have www.domain.com/myapp as a tomcat application, you would do
jkmount /myapp/* worker1
Where worker1 would be defined in the workers.properties file like:
So the request will come in to apache A, which will get proxied to B and then mod_jk'ed over to the tomcat server.