I think this is what you're saying:

You'll have both a cable modem and a router at each house.

In each house, you'll have traffic flowing [interwebs] <-> [cable modem] <-> [WANint/router/LANint] <-> [house LAN]

You then want to run a dedicated line between each house's router so the two house LANs talk without hitting the (otherwise slow) internet.

Fun problem; I'm going to ramble a bit:

First, quick, easy answer: if you just want to access the two servers, you can connect the LANs without any routing and bind two IP addresses - one from each LAN - to each server. Problem solved.

If you want full routing between two separate LANs, things get trickier.

If they are real servers, you could use them to route - connect them with the fiber and point the router's static route to the local server; the local server routes to the remote server and vice versa.

I've mostly done vpns lately, but using traditional IP routers, you'd assign an ip address from each house's LAN to each router. For example, Router 1 would have both x.x.1.1 and x.x.2.2; Router 2 would have x.x.2.1 and x.x.1.2; a static route on each would point to the other. Unfortunately, the manual doesn't seem to show that the Asus router supports this; most consumer firewall/switches won't.

What hardware are you going to use for the fiber connecions? Will it support multiple IP addresses/subnets and routing?

This would work if you have a box with two routable LAN interfaces. They tend to get pricy, though.

Alternatively, if you use a firewall with a DMZ port, you could probably route through them but with two runs connecting the houses. [DMZ1] <-> [LAN2] and [DMZ2] <-> [LAN1] Each router thinks the other's LAN is really the DMZ. (The Asus seems to have something it calls "DMZ" which isn't really; it's just forwarding all inbound internet traffic to an inside IP address.)

You might be able to set it up using vLANs.

This gets a bit weird: you could use the same subnet, but have each router's dhcp server give out the local gateway and have a different pool of addresses. Keep your 192.168.1.0 and 192.168.2.0 ranges, but use a 255.255.0.0 subnet. All static addresses will route out the proper gateway just fine. If a router was a bit slow responding to the dhcp request, the other house's router might answer first, and you'd send internet traffic through the wrong cable modem. If this occasional traffic issue isn't a deal breaker, it would work.

-jk