These points may help:
Changing to more then one wire will require you to run new PQRs.
Most fillet welds on bridge girders are 5/16 inch; do you really need a twin wire setup to make this size fillet? In theory your travel speed would be very fast, creating less distortion and taking less time, which is good, if you have welders that can handle it.
Twin wire for groove welds would be good, but again you have to run new PQRs and it would only really be advantages if you can weld complete slabs, not just cut flanges together. For twin wire or multiple wires in groove welds it would be better to use a side beam, which can be moved around with an overhead crane.
Just my thoughts on the idea.
Peter Kinney