Well done for separating them. As the others have said, sorry to say but a rotostak isn't a good environment for a hamster - however many units! They can't have normal behaviours in there - they need at least 3 to 4" depth of substrate and space to dig and forage and bury hoards. It's tricky if you already have a lot of units as they are expensive, but the best solution might be to make a bin cage and attach the rotostaks to that via the tubes - that way they have a suitable "home" cage - the more substrate you put in it the more they can dig their own tunnels. Or you could get something like one of these cages, which is a suitable size (I wouldn't go any smaller) which has tube attachments already on the cage - the tubes may be a slightly different size but you can probably make them fit.
https://www.amazon.co.uk/Ferplast-57...ast+criceti+15
Savic Hamster Heaven Metro Cage | Great deals at zooplus!
Having said that - external tubes aren't popular for hamsters really - they are an escape route (they can come apart) and encourage nesting and peeing in tubes. With one cage with larger floorspace you can have tubes and tunnels as internal floor toys and they still get the benefit of the whole cage, with more space to move around in and forage in, and lots of hidey places and a shelf to sit on
So I would be tempted to forget the rotostaks. Maybe sell them (I believe they are sometimes suitable for mice, but really they're not that suitable for any species of rodent!). I started out with one - 3 weeks later we upgraded our hamster to a cage the size of the hamster heaven and he was much happier (once settled in).
There are cheaper good cages available which don't have tube adapters. I think once you've had them in a standard cage you will think the Rotostak units were awful! The ventilation is bad with them too.
The standard duna multy is a good size for a single dwarf hamster (although some people go much larger than this) It costs about £33 and has good access for cleaning and narrow bar spacing so they can't escape, plus plenty of depth for 4 to 6" of substrate. You actually end up using less substrate (floor bedding) when you have it deeper because you just need to spot clean the soiled areas mainly and the rest stays dry.
Cage Duna Multy
Two of those would cost about the same as the Ferplast criceti with the tube holes. Or a bin cage made from an Ikea Samla bin (costs about £10) is a popular option if you don't mind a bit of diy.
Here are some examples of dwarf hamster set ups in a duna multy
https://www.google.co.uk/search?q=du...kOCwIQ_AUIBygC
Hammies also need a good sized house or hide that is dark inside, to build a nest in and that is difficult to fit in a rotostak. You can even get labyrinth houses which have separate rooms for nesting, hoarding and weeing (which fit a potty inside).
https://www.amazon.co.uk/Trixie-6201...abyrinth+house
Rodipet make good hamster houses with lift off roofs so you can spot clean inside without the nest falling apart.
https://www.rodipet.de/shop/haeuser/rodipet-granit/
Or you can make a good dark house/hide out of a shoe box with a hole cut in for a door and a bendy bridge tunnel over the door.
This cage is also popular and not expensive and a good size for a dwarf hamster and not too high
http://www.littlepetwarehouse.co.uk/...helf-grey.html
Depending on how big the dwarf hamster is they usually need a 6.5" to 8" wheel as well so they can run properly.
It's hard if you have a big set up and are attached to it, but it would be better at least if they had a main cage that has continuous floorspace and a good depth of substrate, even if you attach the rotostak to it.
I went through all this with our Rotostak unit wondering if I could add the rotostak cage to it it - but that is still too small (and expensive for what it is). So I just bit the bullet and bought an 80cm by 50cm cage