Transparency between WAN and LAN

I have a question regarding remote "workgrouping".

Basically, for geographical and logistical reasons, I have a network within a network. I am wondering how to file share between them. Both networks are common setups with wireless 4-port routers. The router for the second, "inner" network is a D Link DI 524, and currently gets its WAN IP address from the Linksys WRG54GL router on network A.

I have this setup in place because: it's a large property and the broadband comes in in one building, gets shared around to a few proximal buildings (all computers on network A), and the broadband also gets piped down to another building, which has the existing network B.

Now, I'm not concerned about visibility between the two networks. I like the current setup because network B, the "more important" office building, is not dependent on network A, the less important residences (well yes, B depends on A for the broadband, but alas, we could only get the signal from one place...).

(Incidentally, there are only one or two computers on each network that it would be great to share to and from.)

A search revealed the Filezilla ftp solution and port forwarding. Now, a static IP address for router B would be easy to achieve, but I wonder if there is an easier way. Really, it would be ideal if computers from both networks could be on the same workgroup. Is there a way to simply "open up" network B's router to allow outsiders to get in freely somehow and join the workgroup? (How about the DMZ? Also, I assume such an "opening up" wouldn't be a security risk since the whole setup would remain behind the firewall of the primary router.)

Or how about this: Lets say all computers (currently on networks A&B) were on the same network, and had static IP addresses. Obviously this would allow for easy sharing--no problem. My next question is about the apparent dependency--if the "primary" router (currently network A's) went down, say due to a power outage in building A, would the network functionality in building B go down even if all computers in there were on the same switch/wireless access point (the D Link router)? If it stays intact then that approach might be the solution. A bit of a pain to assign all those static IPs though.

Any ideas?