If you really want two bridges or two floodgates or two doors for something, you can set up a water reactor next to two pumps on top of each other (like a pump stack, but they're not connected to each other directly). Each pump has its own powering gear connected to the same lever, and the lever is pulled after the first gear is connected. Put two pressure plates into the water intake tiles and make sure they're set to activate at the same water levels, and the system just needs to be linked to something and the water added.
Note that because pressure plates remain active for 100 ticks after they stop being triggered, the buildings they're connected to won't do their respective things at once. There are a couple of ways around this. One is to hold the water somewhere else for 100 ticks so that one plate deactivates as the other is being activated and the signals go out at the same time. The other way is to use one door and one floodgate. Because floodgates have a delay and doors don't, pulling the lever one way will deactivate the door plate just as the floodgate is responding to its pressure plate. Pulling it the other way will result in a 200-tick delay between the door and the floodgate's respective responses.
Having both a floodgate and a door connected to each pressure plate would result in a system which takes 200 ticks to respond fully to each lever pull. For the first 100 ticks, only the door attached to the just-activated plate has switched states. At 100 ticks, the floodgate attached to the just-activated pressure plate responds and the just-deactivated pressure plate switches off, causing the door connected to that plate to change states and sending out a signal to the connected floodgate. At 200 ticks, the floodgate connected to the just-deactivated plate responds.
This would work out well for trade depot airlocks, since having the depot sealed off from both the fortress and the outside world for a little while is no big deal.