I love the Barney cage! The Maxi Duna Multy is great too. They are both 100cm cages - so will only be 20cm wider than the Hamster Heaven, but the Barney does seem to create a lot more floor space because of the extra height, meaning you can put somethings on shelves - and have a bit of a roof run as well.
You are right - one larger floor area cage would be much better than two smaller areas. Not that the Hamster Heaven is small but it's the minimum size for a Hamster. So one 100cm cage would be better than two 80cm ones.
This is the Barney on top of Charlie's old cage which was the same size as the Hamster Heaven (it was just there while I was switching him over). As you can see, it's only 20cm longer but somehow a lot more useable space - it has a good deep base and is also deeper than the Hamster Heaven (about 54cm deep) as well as longer.
Could you fit 120cm in? These are great I reckon - a bit expensive though
https://www.etsy.com/uk/listing/2535...?ref=related-3
The Alexander cage is also good - it's a bit taller than the Barney and has almost a full level in, but all in one cage, so similar to having two on top of each other but more floor space and all accessible. It's about the biggest commercial cage you can get in the Uk.
Great deals on small pet cages at zooplus: Alexander Small Pet Cage
Or there are also the Living World Eco Habitats. The 100 cm one is a good size because you get the full floorspace without the curved base of a cage, and it's also about 55cm deep. There's a 120cm one as well that's about 75cm deep. The medium size is the 100cm one. Not cheap either but a lot cheaper than a fish tank.
Living World Green Eco Habitat | Free P&P on orders £29+ at zooplus!
The downside is the vents on top are 2cm so could do with meshing really.
I would go with a Barney or Alexander cage. Except if you are concerned she might still chew the bars in a bigger cage (is she chewing them or just rattling them?!) then a tank style would be better, in which case.
Maxi Duna Multy
Eco Habitat
Second hand 4 foot fish tank (they can be good value but sometimes hard to find)
Sometimes with more space, as in the Alexander cage, they don't chew the bars - especially with narrow bar spacing like in the Alexander or Barney.
This is Charlie's Barney cage now it's set up and there is loads more room for toys and still plenty of floor space.
Before (Savic cage that's the same size as the HH)
After (Barney cage - mine has different coloured bars but it's the same cage)