For some reason the water in the tank is not getting to temperature. As you use the water, cold water enters the tank, so you get lukewarm at the faucet. Then the heater kicks in and starts warming the water.
1. If electric, could be the second thermostat. 2. Could be too much crud in the bottom of the tank, particularly if its an older tank. Has it ever been drained to flush out the buildup (too late now - needs to be done on a regular basis). The crud buildup insulates the water so it doesn't reach full temperature.
As for the second bathroom - how is the water pressure? I had this problem years ago and it turned out to be hot water side of the mixer was blocked with corrosion buildup.
BTW - do you have hard water? Use a water softener and is it working?