Bridging two VPN tunnels

I am at the central office with a FireBox X750e and has IPSec tunnel to branch Office-A, and one more IPSec tunnel to Office-B.

I have a web application on Office-A that should be made available to the users at Office-B. There cannot be a tunnel between Office-B and Office-A for some internal regulations.
I request some one from this forum to advise me on this tricky scenario. Thanks.