Lovely names - Rhubarb and Crumble - and Ruby
Great to hear they are friendly with you and doing so well
I think they often prefer running on a wheel together but it's still important to have a second wheel so they have the option to run separately and don't start fighting for dominance over the wheel. Sometimes a syrian sized wheel is good for them to run in together as it's deeper with a smaller wheel for the second wheel. You could even have 3 wheels or 2 and a flying saucer
Then they have lots of options!