Re: Betwixt and Between Layout - I have seen the Elephant
Are the two bascule bridges joined together when closed at a center pier? If so that would be an ideal place to use a centering pin on one side and a V shaped mating part on the other to ensure mechanical rail alignment as the bridges close. Make the V shape mechanism alignment adjustable as a precision and tight fitting is required, particularly for N scale.
Either way... If a train is wanted to traverse the boundaries between two supposedly separate layouts without any DCC electrical interference or interruption, both layouts and the bridge tracks themselves between layouts must be running in DCC sync. This means that somehow the DCC control signaling on both layout halves must be the same whenever the bridge(s) are closed. You could use ONE central DCC controller to operate both layout halves. Each layout then simply becomes a separate 'booster district'. No electrical issues at all. You can use relays or switches to temporarily turn DCC power boosters to one layout side or the other OFF when wanted for servicing. Take your pick of controller brands and models.
As discussed earlier if you insist on using two separate DCC controllers, with separate boosters, it may be possible to use a relay to flip the DCC layout power source to the bridge tracks from one DCC layout side to the other as a loco travels over the bridge. [Requires which-way direction logic info] But remember to coordinate the loco address and speed setting between to the two DCC controllers to prevent loss of loco control. However, all instantaneous settings must be the same on each layout for a smooth transition. A central computer (or a single DCC controller operating both layout sides) can be used to do that. Re-read the simple solution above.
One alternative is to jump to direct radio controls with DCC power on the rails. See RailPro by RingEngineering. https://www.ringengineering.com/RailProVideosPage.htm These units steal loco operating power from the rails and can be used simultaneously with DCC. However each RailPro loco moving over and between separate DCC layouts is essentially radio controlled by 'their' operating box. Somewhat impressive but of course works with only their equipment. An interesting way to swap cars between layouts. Not sure how to power bridge tracks and not cause interference with DCC power boosters on each end. Re-read the self imposed complications above. Not sure how RailPro receiver/decoders will fit into N scale.
Another alternative would be to skip DCC altogether and use DC. But again the adjustable DC voltage on each side as well as that on the bridge tracks must be the same voltage and polarity for a smooth transition. All of the older DC layout track district/section and controller/throttle issues will still be there.