Nowhere does the system prohibit you from having two or more slaves on the same channel, either. A number of my crossings use multiple slaves on the same channel to be able to delay gates dropping or rising, (or holding carz at the tracks for a few extra seconds so they stop T-boning my last car! lol)
A prototypical sceanrio where this multi-slave delay would be desirable would be for full-quadrant gates such as this video -
https://www.youtube.com/watch?v=ee93yveCDGM - You have a slave with 0 "start" delay running your right-handed gates, but the left handed gates are run by a second slave with a delay to avoid trapping vehicles on the tracks.
Not that its what you're looking for, but someone might find this useful - an even trickier scenario I toyed with (and surprisingly worked without invisible train collisions) would be for a crossing such as this -
https://goo.gl/maps/5s3wO - where two slaves on different channels run the same gates - In this image specifically you have the NS Lake Erie District on the single track, and the CSX Lakeshore Subdivision on the triple tracks. Because of the proximity of the crossings and frequency of trains on both main lines, you have semi-interlocked crossings - if a CSX train approaches, both gates on that track will lower but the gates approaching the "outer" side of the NS line will also lower to prevent traffic from stacking at the CSX crossing and potentially blocking the NS crossing, but the inner gate at the NS crossing will remain up unless an NS train approaches as well. This also obviously happens in reverse if an NS train approaches first, etc.
I set this up on a proto route I was working on at one time by having an individual slave work the inner gates for each set of tracks, but having a second slave from both track channels connected together to the outside gates and the traffic stoppers. The only downside was that traffic would obviously stop in both directions at both crossings each time, but I probably could've fixed that had I felt like dealing with one-way invisible road through the area as well.