I've bought two routers with built in VPN. The idea is to be able to securly reach one LAN from another LAN .

So, if I do this (as an example):
192.168.1.xxx -LAN- (192.168.1.1)VPNFW(1.2.3.4) - I-N-T-E-R-N-E-T - (1.2.3.5)VPNFW(192.168.2.1) -LAN- 192.168.2.xxx
(Two VPN firewalls talking to each other over internet)
...all is fine. Works like a charm.
But now I have run in to a situation where I need to do this (as an example):
192.168.1.xxx -LAN- (192.168.1.1)VPNFW(10.1.2.99) -LAN- (10.1.2.1)FW(1.2.3.4) - I-N-T-E-R-N-E-T - (1.2.3.5)VPNFW(192.168.2.1) -LAN- 192.168.2.xxx
(Two VPN firewalls talking to each other over internet but one is behind another firewall)
1) How do I do this? (Supposing I can get a static IP for "my" VPN FW)
2) What if "my" VPN FW gets dynamic IP from the firewall it is behind?
Ideas of my own.
I) Try to put my firewall on the other firewalls DMZ. This however might not be possible.
II) Let VPN traffic through the other firewall, making a (routing??)-rule that says... Well what?
III) ...

Any ideas?
Regards
/Jens