Cant see a problem, I have two EX48s and one LX36 stacked, one for my carpet python and the other for two of my corns, out of interest what are you going to keep in the 55?
Call me a pain in the bum for bringing this up again but just keep an eye on stacking smaller vivs on top of bigger ones. They're normally strong enough to take it but there is chance of sagging in the middle, especially if humidity is high. I've not experienced this myself but I have seen other pieces of furniture do it and have always been told to avoid it where possible.
Just posted on a similar topic.I have 2 LX 36 stacked on a custom 24" and although it is solid and stable, there is just enough bowing to prevent the doors from being removed. Any mixture of viv sizes whether large on top or 'twins' on top needs a bit of midway support. Could be as simple as a broom handle prop. Ideally the two half sizes should be bolted (not just screwed) together with 1" spreader washers. This will make it more rigid and at least reduce the weight force on the mid-point below.
As you are intending using 4' vivs, this problem will be more noticeable. I would consider bolting the stack between levels as well as the twins.
1.0.0: Miami Normal: Taggart 0.0.1: Carolina Normal: Wee Woody 0.0.1: Grey Rat Snake: Mirkey 0.0.1: Amel Corn: Blaze 1.0.0: Carpet Python: Vambo 0.0.1: MBK: Fester 0.0.1: Florida King Snake : Rattler (Rattle & Hum) 0.0.1. California King Snake: El Diablo Diego 0.2.0 Tarantulas : Ermintrude & Euphemia.