I went through all this when there was a litter of robos up for adoption. I would have loved to have a pair - although you always have to be prepared to separate them quickly. But got in such a twist trying to work out what cage would work best and the set up with no levels etc, and in the end there was only one boy left so I got him.
It was much easier setting up a suitable cage for a single robo
Your tank would be fine for a single robo. Personally I would also go a bit bigger for a pair. I was looking at 90cm by 45cm for a pair.
And yes they love digging tunnels. At least 6" of substrate is good. On the other hand they can hide away too much then so it makes taming and interacting hard. What I had in the end was a labyrinth house which mimics a burrow and rooms etc, and our robo moved into that and stopped bothering to dig tunnels - although he did have another "emergency" burrow near his wheel.
If you want to get a pair then I would get a bigger cage/tank. And plan the set up carefully. It's difficult as I know some breeders feel that robos do better in pairs and groups and need company - despite needing separating sometimes. But our robo has done fine on his own. There are others who say they shouldn't live in pairs and groups.
The only way you can have a pair though is siblings of the same sex. If you were having a group of 4 eg, then a 4 foot tank is a good size (as per robo breeder advice I got).