Wet Wall with plumbing on both sides
Sure, it's done all the time.  Thickness of the wall will likely be determined by the required vent size.  With two toilets, two tubs and four sinks, I think you'll likely have to go to a 2x6 wall if you go with a single vent stack, but you'd have to run the numbers.

I did run into a code issue with drains T'ing into the main vertical stack at the same level.  Still not sure what the issue was, but the made me change it.

As far as accessing it later.  If everything is properly pressure tested before you close it in, you should be fine.  But stuff happens.  The odds of it happening are inversely proportional to how easy it is to access the area.  So if you had an access panel there, it would never leak there anyway.  Mostly joking, but seriously - if you have a problem, you may have to break into the wall.  No fun, but unlikely and not the end of the world.