Syrians need good sized cages and plenty of floor space. I'm not too keen on the Rotostaks having had one and upgrading to a larger cage - the ventilation isn't very good apart from anything else. You'd need all the units on one level for the dwarf to use the tubes, but Dwarf hamsters also need a lot of floor space, like Syrians, so I think you'd do well to invest in a new cage (or two?).
These would be good cages for a syrian or a dwarf hamster:
Cage Maxi Duna Multy
Cage Maxi Duna Multy
This one would be the absolute minimum size suitable for a Syrian or Dwarf hamster
http://www.amazon.co.uk/Riviera-Vara...0+x+40+x+40+cm
This one is a budget option for a syrian - meets minimum recommended size. It wouldn't be good for a dwarf hamster as the bar spacing isn't narrow enough.
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And this one would be about the best you can get for either a Syrian or Dwarf Hamster - loads of room and two levels. You could still use some of your Rotostak tubes inside the cage as toys for the Dwarf Hamster. Our Syrian got stuck in Rotostak tubes, but the curved ones are a bit better than the straight up and down internal ones.
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If your other cage (non Rotostak) meets the minimum recommended requirements of 70 x 40 cm floor space (I think it's actually 80 x 50 now) then I would keep that one for your Dwarf Hamster and get a new cage for the Syrian. You could sell the Rotostak units on ebay maybe (apparently they're ok for mice), or keep it as a temporary cage or digging bin for when you are cage cleaning.
I can't see a Dwarf Hamster being happy in a rotostak - they are very active creatures and need lots of space. And I definitely wouldn't put a Syrian hamster in a Rotostak.
We made an expensive mistake getting one but it's the fault of manufacturers marketing these things as suitable for hamsters. They'd be a great idea if they were 3 times bigger and had better ventilation.
Have you got a pic of the non rotostak cage and we might be able to advise.