2 x windows 2003 server on seperate subnets... domain trust? failover?

[realized]

I have in california a windows 2003 standard machine as my only domain controler on its own domain. Its on a 192.168.111.x network

IN another part of the state i have a windows 2003 standard machine as the only domain controler (different domain then above), on a 192.168.222.x network

I have a pinned up VPN between the two for simple backup scripts that i have to ftp info back n forth. The VPN is pinned up via two watchguard fireboxes.. the VPN will allow or not allow any kind of traffic - this is not an issue.

I would like if a server in one location went down, for the other to pick up the slack untill repaired..

Is this possible? Do i add them to the same domain? can they replicate between the two on seperate networks? can AD replicate and take over if one went down?