Everyone else has covered all the points londonhamster and you have definately come to the right place for advice. Everyone on here has wide experience of keeping dwarf hamsters alone and in pairs/groups, so their advice is invaluable to a newcomer as so many new owners get all excited about their new little ones and sometimes that excitement turns to stress and disapointment when they realise they have either bought the wrong cage or their pair of hamsters start to argue and not get on and end up needing split up. It happens a lot sadly.
Have you thought about adopting one of the russian dwarfs from Pets At Home? If it`s really a pair you are after that`s fine too, but I have to be honest here and say I would never take on a pair of dwarfs again. I just find that they can be too much trouble to keep an eye on and you end up with two cages rather than the big expensive one you bought to begin with!
That`s not to say the two you buy would end up seperated, but it is something you need to consider long term just incase. For a pair of dwarf hamsters, I would be looking at cages like the Zoozone 1, the Ferplast KIOS, or the Zoozone 2, although this is a meter long so you need the room space to accommodate it! Fantastic size though for plenty of cage accessories for a pair or a single dwarf.
All of these cages can be found online and sometimes local pet shops. They would be a good, long term solution and all have decent floor space without having multiple shelving. You can add a few low levels into cages by using wooden levels and Fiddlestix log bridges etc... plenty of ideas in the Housing and Habitat sections on here to give you ideas.
But finally, if this is your first venture into owning a dwarf hamster, I would look at adopting one that is looking for a nice, settled home. I prefer males but that`s just my own preference. I get all my hamsters from adoption or word of mouth/forums etc. It`s so rewarding.