Linux box 2 Linux Box VPN both going through firewalls … howto?

I have a dilema, I have two linux boxes, in two countries, at two offices,
of the same country. Now, they want thier offices to both connect via
VPN, so that they can transparently all communicate, and connect to the
same resources.

Both offices, and protected via firewalls and I can ssh from one box,
directly into the other. How can I get this VPN up and running???