Buffering Residential Water Pressure
Pressure drops are normal. That's just how it works.  Even with much larger pipe, there'll be pressure changes as the pump kicks in and out.   

I would start with a pressure regulator.  Set the regulator for something like 35 PSI which the system can always provide.   It would work even better if both houses had a pressure regulator.   It goes without saying that this will reduce best-condition flow somewhat, in favor of more uniform flow all the time. 

After you have a regulator (which implies a check valve), then a "pressure tank" will start to make sense.  This buffers further.   Although I have a concern with them in light of what we're learning about hot water heaters and legionella.  If it's not good for water to be stagnant in a hot water heater below 140F, one of those pressure tanks has the same basic problem.