I use the smaller Ikea Mixtur (11" x 7") as sand dish and it seems like a good size for pairs. Two can roll in it at the same time with still plenty of space to run around in.
I've found that one water bottle works fine for my pairs (they never fight over the bottle), but it never hurts to have an extra.
Puberty typically start at around 10-12 weeks of age and can last until 6-8 months old. I don't think you necessarily have to keep everything super simple until then (since it's a long time), but you can start out simple and adjust as needed. If you see them start fighting over something, then remove it, etc.