Loads of substrate
25cm to 30cm deep if you can - they can have fun digging tunnel systems. Assume you already know about two of everything. Two houses, two wheels, two food bowls, two water bottles, no levels.
You could make a couple of good houses out of shoe boxes. If you cut the bottom out and cut two holes in the front for doors you can have a house with two entrances (one at each long end of the box maybe) and also a lift-off roof! The lid. That way you can check inside without having to lift the house out and destroy the nest.
They also need some kind of overhead cover or they can feel a bit exposed from above (partly why they tunnel I think). I like the big cork logs you can get in reptile shops. It's plenty big enough for two robos to be inside so you won't get one blocking the entrance and being territorial.
It also gives something to climb over and an interesting texture and will cover a lot of floor space, but is light so won't sink in the substrate. They tend to be a bit dusty and mucky when you get them but just need a rinse and then drying out. It's also a good idea to freeze cork items for a week before use (or bake them) to make sure there are no microscopic things living inside.
These sandblasted branches are also good - something to hide under and run up.
https://www.amazon.com/dp/B00C0PCIZI?psc=1
I also like these balls. Especially the hyacinth one which is good for sticking treats in that they chew at to get out. I'm a bit wary of the wicker one in case it's sharp when chewed but the other two are ok.
https://www.amazon.com/Niteangel-Sma...=trio+of+balls
They can have fun pushing them around and trying to get treats out.
Pairs of robos like snuggling together too, so round hidey place toys could be good like a coconut shell - as long as the entrance hole is big enough - or maybe one with two entrance holes.
So I'd say keep it simple but add to the enrichment with textures and variety. Individual toys could lead to fighting - unless you have two of them maybe