You can ceiling mount two shower heads on each narrow end but keep controls on the interior wall to avoid losing the four inches.

I don't think the 4" is a deal breaker and would much prefer wall mounted heads vs overhead, but thank you for the suggestion.

We did something similar to this design in a previous house. We had a 54x48" shower with shower heads at each end and centered opening. Instead of a corner bench we added a corner "perch" saving $$. If I'm reading your plans right you have 90" to play with right? You could center the shower opening, make it 30" wide and build out the exterior wall 4-6" allowing for plumbing and niches for shower supplies- leaving plenty of shower space. We didn't put a door in, just hung 2 hooks and altered a shower curtain. No post shower squeegee and we could see the great tile work.

If the shower heads were on the long wall, you'd end up with the potential for overspray and/or rebound water to leave the shower through the shower door.

I'd recommend as you wrote, having the shower heads on the two ends walls with the door opening centered on the long wall.

If you put your two shower supply valves on the door wall, one on each side of the opening, it makes things a bit nicer.

If you do that, your added wet wall against the SIP panel can be 2x4s on the flat. The 1-1/2" deep stud bay you'd get with the studs on the flat is more than sufficient for the plumbing run to the shower head. You'll lose two inches less floor space than with a conventionally framed wall.

By centering the shower entry on the long wall, it adds usable space on the bathroom/bedroom wall just outside the shower for a storage cabinet.

A cuple fo comments from the Department of Unsolicited Advice:

Unless they are something you REALLY want, rethink the double doors into the bathroom. Architects and designers love them for the dramatic look. But double doors can be rather impractical for passage ways that get frequent use. Impractical to me at least.

SIPs? Nice choice. You'll love living in a foam insulated house. However, if you haven't already done so, spend a few minutes carefully going over the floor plan. Figure out what you want for electrical outlets, RG-XX, CAT-XX, entertainment, home security, etc, in your exterior walls. You can modify later, but it can be messy and a bit of a pain. If uncertain regarding low voltage stuff, you can always run conduit now to a "home run closet", leave the conduit empty, and pull the wire later if needed. Much less expensive to add conduit now than trying to modify any of the wiring later.

Good luck with the build.