Merge 2 water line to increase pressure
Your pressure drop issue is really a problem with flow capacity. You will not be changing or improving the flow capacity of your main house's plumbing system by tying in that 1/2" line from the guest house and will likely see zero improvement by doing so.

You would be better off trying other things:
Install low-flow water saving shower heads (this will help more than you think).

Check your water meter outlet size and ask your service provider to install a larger one if possible, then replace the main service line with larger pipe (I know, big job).

If you have a pressure regulator anywhere in the system, check it for proper operation. Raise the pressure setting if possible (check with city bldg. dept. for max allowed).