Instead of harming the original, why not clone it?
Build a copy of the sides, and then widen it by customizing what goes between them to accommodate a two-track main. Widen the floor cross-members enough to add a second track at whatever mainline spacing you use, then measure out the diagonals for the upper chord and build to suit
If you need a row of trusses between the two tracks, then build a copy of the single-track bridge, but with a double-wide floor to accommodate the second track. Then, add a third set of trusses to enclose the structure around track two.
At least, the above is what I'd do..
---PCJ
