There’s different foams. The messy one is the EPS polystyrene (Expanded PS)… the stuff of cheap foam coolers. Makes little balls of impossible to clean up crap when you cut it.
XPS is extruded, and a more rigid, no balls. doesn’t need to be faced. It’s the pink stuff at HD. There’s also polyiso, which is rigid, and usually faced. Neither are crumbly or dusty, lol!
The reason for 2x layers is to stop as much air infiltration as cheaply as possible. One layer would do, but all the seams need to be sealed.
Obviously, to do a wall like that, you’d strip the rock, air seal the gaps with spray foam, and put rockwool between the the studs, then re-rock. But, that’s a lot of messy work, unless you want the basement for future living space.
Instead, you could strap the inside wall with 1x2. More airspace, better insulative value. But I don’t think you’d need to, nor would you get condensation inside the wall. I think I’d just tack the foam board to the sheetrock, and seal all the edges. You could always add more.
I’m imagining you have a concrete slab floor, and it’s probably cold. That’s why the insulation board under the tent (s), and the raising the plants off the floor a bit.
I had a little oil-filled radiator style electric heater that would take the chill off, but my space is large and would be a bitch to section off, and I’m cheap, so I just let the plants ride it out through the winter… which isn’t too bad in NC. The plants don’t seem to mind either.
Hope that didn’t muddy the waters even further