Hello and welcome to HC!
A 20 long is quite small for any species of hamster unfortunately. I'd highly recommend upgrading them all to a larger cage if possible. Syrian hamsters do best in cages of 1.55k sq inches or larger, but can suffice in smaller cages if you can provide enough stimulation and out of cage time. Your WW's should be kept in a cage of around 600-800 sq in if they're a pair (as I would assume). I'd like to note though that pairs are not recommended as they're quite risky for all hamsters involved. It also takes quite a large amount of special care to keep two together.
I keep some some of my cages on shelves in order to free up floorspace and they've always worked just fine. As long as you're sure the shelves can hold 30 lbs or so, you should be okay to put them on there. I think the idea you have would be fine, but I personally use wire shelves. Here's an example of what I use
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Good luck, and if you ever have any other questions, please ask