You best bet may be to keep both links, and (either with DHCP or static configuration) give machines in each building their "local" router as a default gateway. This avoids Chopper3's highlighted issue whereby people could lose Internet access if the laser links fail, and gives you very low-tech load balancing to utilise both connections.
Alternatively, if you have a service that tends to saturate your ADSL uplink (say), or a service that requires low latency (VOIP?), you might choose to just route specific services over one link -- for instance, at one of our sites I have VOIP handsets talking to a controller at our "main" site, these are on the same subnet as all our PCs, servers etc I have 2 ADSL routers, as
192.168.0.254, VOIP goes through the former and all other Internet traffic through the latter.
A final option would be to use one link as the "primary" and fail people over to the other if it goes down, by switching its IP to their configured default gateway. This would give you again a fairly low-tech approach to increasing the availability of ADSL, which ... well your mileage may vary, but I'm always disappointed :-)
Really it depends what your needs are -- do you need to increase bandwidth/availability for your users in general, do you have specific services that need their own private lane for traffic (either to avoid impacting others, or to avoid being impacted), or are you really just OK as you are.