Couldn't you just simply physically lock the one slide in place (bolt through the track or something?) so the fluid/pressure would move on to the next slide in line...same as if the kitchen slide were the first to fully extend? Seems like you could test this fairly easily. If effective, this would eliminate the need to tap into the hydraulic lines.
When shopping for trailers, individually activated slides was on my wanted list....this usually means they're electric though...NO idea if that's a good thing or a bad thing.
'04 GMC Sierra 2500HD, CCSB, D/A, MaxBrake, B&W Companion, Retrax Pro, AR Power Steps
2013 Rockwood 8289WS w/Diamond pkg
1969 Camaro RS/SS Convertible
Dad's old 1968 Empi Imp (recently recovered from under 21 years worth of boxes/dust)