Systems can utilize multiple bridges. The systems just have to have a capacity rating high enough to accommodate the weight if both bridges we MAX loaded. This is where it gets a little complicated.
If you want the two bridges to be able to travel in the same bay, the bay must be rated to accommodate both bridges MAX loaded.
If you have multiple bridges, however, and stop the bridges from entering the same bay together (In other words, you will never have a bay with more than one bridge in it), each bridge can be rated at 6,000lbs (if we follow the two above examples). Bridges can be stopped from entering bays by installing what's called "runstops" inside the runway. Runstops prevent bridges from traveling any further.
see pictures at http://ping.fm/Ilxad
