Hello! If you aren't aware, in the UK, the cage minimum is 589 square inches (or 80 x 50cm). However if I were to adopt two hamsters I would probably double this or if you like you can go in between. If you would like to double the size I strongly recommend a detolf tank as these are great for robo pairs. They measure just over 1000 square inches and are easily divided with one of the glass shelves if they have a fight. It is originally bought as a cabinet from ikea however, turned on it's back with no shelves or door added, it makes a perfect and very popular habitat.
Compulsory Supplies
As for the contents of the cage, you should firstly have your substrate and bedding (no fluff/cotton bedding). Next you will need either none, one or two food bowls. If you prefer to scatter feed (sprinkle the food around for them to find) then this isn't needed. If you prefer bowl feeding they can share or, to make sure of no fighting over food, two bowls. Also you will need two water bottles most probably as one ham may guard the water bottle, preventing the other from drinking. Next you will need two or more hideouts, they will probably sleep in a hut together but it is better for them to have their own space to retreat to. Another crucial supply is a wheel/flying saucer. You will need two, although they may run together, they could want to be alone at some point. A good size to go with is 6.5". You will also need a variety of chews, a good mix (I recommend Harry Hamster or Burgess Hamster Harvest),
toys, cleaning supplies and probably a carrier.
Optional/Recommended Supplies
A sand bath is highly recommended for robos as their fur is naturally quite greasy. All you need is some chinchilla sand (not dust) or sterilised play sand in a container for them to roll about in. Many hams also use it as a litter tray! A room temperature thermometer is a good thing to have, costing around £5 or less on amazon, it can help you see if it is too cold/warm for your ham so you can cool/warm them down/up. Digital scales are great as weight gain/loss is usually the first sign of illness. Airtight containers/jars are a great thing to have as well to store food, treats and even sand. Treats are great, but preferably natural things like the woodlands range, forage bags, nuts, seeds and vegetables. The last optional supply is a first aid kit for emergency injury or illness response.
Good luck with your robos, hope I helped!