Have a look at the Lets see your cages thread for some ideas
It is a total pain when you have bought two cages and now need another! I had that too - spent a fortune on a useless rotostak thing with add ons etc and within 3 weeks realised it was giving the hamster cage aggression and ventilation issues - and found this forum and bought an 80 x 50 cage - it made a huge difference to our hamster's personality. Ended up binning the rotostak - an expensive mistake but it is what pet shops sell you sometimes.
Technically two cages joined together is fine - if one of them is at least 80cm by 50 cm - but it is much better to have one good sized floorspace. If you think about it, it would be like having to climb through a tunnel between your kitchen and living room every time to the point you probably stopped bothering and it was annoying!
I've added a photo of our Hamster Plaza cage set up for a Syrian who we lost a few months ago - he died of old age. In it I have a large labyrinth house (at one end - it doubles up as a shelf) and bought a wood shelf for the other end (which needed cutting to size a bit) - as that maximised the space. The large green shelf that comes with it and its long ladder, make it difficult to fit other things in.
There is a rat tunnel tied to the roof which leads into a rat sputnik. All our hamsters have loved hanging out in their rat sputniks! Providing its easy to get to - either via a tunnel or by stepping in from a shelf. The tube and sputnik also give a kind of "roof run" - somewhere else to go - while also giving overhead cover.
The big front door is brilliant on that cage and you can easily interact with a hamster plus they like approaching the big open door and looking out.
Final tip - freeze the bags of hamster food for a week before use - it helps prevent moth outbreaks (pantry moths).
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