Plumbing Questions? Ask a Plumber and Get Answers ASAP.
Welcome to Just Answer, my name is ***** ***** I will do my best to help you with your issue. If my initial response doesn’t answer your question then let me know and we can continue our conversation.Do your shower valves/faucets have 1 handle or more than 1?
Ok, it's possible that the temperature limit stop is set too low. This simple device stops the handle from traveling too far in the hot direction to reduce or eliminate the chances of scalding. If it's set too low then the handle never moves far enough for the water to get hot enough. The other possibility is that the cartridge in the valve (you probably call it a faucet, we call it a valve) is failing and needs to be replaced.
If the limiter is not the issue then the only other possibility is the cartridge assuming you have no hot water problems anywhere else in the house. It's possible when the work was done sediment could have fouled the cartridges. When the water is shut off then turned back on again after a repair it stirs up the sediment that clings to the inside of the pipes. So flushing out the lines would be a good idea when you have the valve apart.Actually there is one other possibility but it's so remote I didn't mention it. That would be something restricting the hot water flow to these 2 showers. That is only even possible if they are both fed by the same branch line
There's no way to know for sure unless the pipes are exposed (like in a basement). Unless the showers are back to back there's virtually no chance they're fed off the end of the same branch. To flush the valve you turn the water off, pull the guts out then turn the water back on. This is a lot easier if there are screwdriver stops at the valve but in most cases there aren't then this is best done with 2 people. One to turn the water on and off, the other to be sure the water stays in the shower.
Like I said before shutting off the water stirs up the sediment in the pipes which can foul the cartridges. Since you're not on a slab the piping restriction is even more remote. The shower pressure issue could be due to clogged or not very good shower heads.
I'd try flushing one of the valves and change that cartridge. If that doesn't solve the problem then you have supply piping issues which is not an easy fix on a second floor bath.