Low water pressure in duplex

[Devin Radford]

The duplex is on a shared water supply. Any time two or more fixtures are being used at the same time the water in one becomes non existence immediately.

Each side of the duplex has its own full bathroom, water heater and kitchen sink.

The duplex is on a concrete slab.

I cannot find the shutoff valve (except for the city shutoff valve) or the prv (I'm assuming there's not one).

The water supply line is 3/4 and cutting a hole in the wall I discovered the lines feeding the fixtures are 1/2.

City has came out everything is good on their end and they place a pressure gauge on external tap and said its low

I checked faucet aerators

the 3/4 main supply is cpvc and everything else is copper

The water bill is not high so I assume there are no leaks

A plumber is suggest converting the lines that are 1/2 to 3/4 until it gets to the fixture. I am a newbie so i wanted to get advice before making pricey decision

Other Plumbers are suggesting to add a prv they said " this is usually the solution"

Again I am a newbie but if a prv is added that would only control the water pressure not increase it, correct???