Tunnels would be nice, though. Make them expensive and maybe having to cross one would have a comfort penalty or something.
If you are in dire need (e.g. an injured person needs a clinic), you can re-assign colonists between those domes, but they will not use the other dome's internal facilities by themselves.
I assume that is mostly a gameplay related design decision. If it wasn't the case, larger domes would be significantly less useful.