I'm adding a second bathroom back-to-back with the existing bathroom at my house and I'm trying to understand how I plumbed the additional bathroom incorrectly because I'm losing pressure when I turn both showers on at once. Note that I'm losing more pressure when I turn on the hot water than the cold. Here's my setup:
Cold water supply
I have a 1" copper cold water supply line coming into the crawl space of my house from the meter (I don't know the exact pressure, but the city I'm in says it's an average of 53 psi city-wide). Cold water then branches from the 1" pipe to a) the hot water heater with a 3/4" cold supply line b) the kitchen with a 1/2-inch line c) the existing bathroom with a 1/2" line. The 1/2" cold-water supply to the bathroom is roughly 30' in total distance from that 1" main supply line if you add up all the 1/2" piping to the shower, bathroom faucet and toilet (there are multiple 90-degree turns en route to these fixtures, but nothing out of the ordinary I don't think, though I think more turns means more pressure loss).
Hot water supply
As I said, there's a 3/4" cold water supply line that goes to the tankless hot water heater. Then the hot water exits the tankless in a 3/4" pipe as well. About 6 feet from the heater the hot water tees to a) the shower and sink in 1/2" supply lines running roughly 20 feet in total to those fixtures (again with some 90-degrees en route, but nothing seemingly out of the ordinary) b) the kitchen with 1/2" supply lines running a fairly long distance of perhaps 75 feet or so.
I thought it would be ok to tee off of the 1/2" line that goes to the existing shower/bath to run 1/2" supply lines to the new shower right "behind" the existing shower. However, now that I've done that, when I turn on the existing shower/bath, and then turn on the new shower, the existing shower/bath pressure drops down significantly ("significantly" is based on sight because the shower itself just loses a ton of pressure when you look at it). Same happens with the cold water, but not nearly as much as the hot.
Last comment that I think might be relevant: when I say I turn on the new shower, what I'm actually turning on is a non-flow-restricted "compression" fixture. Is that perhaps what's causing this pressure drop in the existing shower? That compression fixture cranks water out at roughly 6 gallons per minute (I just filled a 5-gallon bucket nearly the entire way in 48 seconds to test it out). Maybe the hot water heater can't keep up with that? I have a Navien 240a that I think does 8.7gpm, which would be close, but still short, of the total number of gallons I'm using in my test here (1.8gpm for existing shower and ~6gpm in the compression fixture).
If that last comment seems really dumb and something I should've shared at the beginning, my apologies. If it's the compression fixture that's causing all of this because it cranks out so much water and so the issue will likely be resolved when the 1.8gpm shower (I'm in California) is installed thanks for taking the time to read this far and correct this ignorant amateur plumber.