I can't say I have run it this way, but we did mock it up at the local hobby shop.
#1 I assume we are talking the tinplate versions, either Lionel or MTH repro, or an orginal VS say the MTH plastic remake.
#2 So in that case of the tinplate bridges, the plastic insulator spacers are made to go into tubular track ties and yes, on the ones I touched (2 different recent bridges) those are made with prelocated holes for 2 lines of tubular track. You can put fasttrack down it, just that spaces up the resulting rails slightly giving you less overhead clearance. What I cannot remember, was the spacing if 2 fastracks side by side how that spaced out or fit.
#3 The biggest problems I know of with this style of bridge or any bridge really: Ensure you have plenty of straight before the bridge and after the bridge for boilers to swing, long cars to align and so forth. Far too often people try to slap a curve right at the entrance or exit to a bridge.
#4 TMCC radio signal. The lights on these are using the bridge frame as ground, so if you use the same transformer accessory power for the bridge or some sort of common "U" terminal, you can affect your TMCC signal across the bridge and trains will stop dead in the bridge. One way we handled this locally was either not use the lights (not ideal) or use a different small like a 20 watt K-line accessory transformer just for this bridge lights so as to isolate it from the tracks common U and thus TMCC signal.
Sadly, I have a brand new MTH repro in the box not yet on my own layout mainly because thinking about doing a tinplate only layout and given I also have standard gauge, making more 5 rail standard and O combined track for said tinplate land.
I'll try this morning/afternoon to unbox and take pictures of various track options for you. Again, I've used this bridge, I've helped others with this bridge, just don't have a recent example with pictures of a double track and the "how we did it".