Cum hoc ergo propter hoc.

Translation: with this, therefore because of this

Relevance: It's a logical fallacy. As such, in your situation, the faucets, whether single or two handled, are not the cause of the problem.

What probably is the issue is that the master bath shower and kitchen have properties that make them the hardest to get hot water. Could be the total linear feet of the supply lines, the fact that the lines may pass in front of a vent, the lines may not have foam insulation, different pipe sizes/material, etc. Basically, those two locations are different than the rest of the house - the only way to know what's up, is, get under there and take a look. It's fun!! You get to meet all sorts of interesting beings - give it a try!

I appreciate the response. We have a full clear view of the entire pipe work - full size basement unfinished, so it is not as bad as most people who have to deal with croll space.

The lines all run through joists but none are insulated - the lines are just below the insulation. The main lines are 3/4 PEX that drop down to 1/2 PEX for each fixture. They are all on the same level/height. The kitchen is very close to heater - utility room is next to kitchen. The MB is in the furthest opposite end of the rancher.

Just take MB in consideration -- it has three different fixtures - two sinks, one bath tub, and a separate shower stall. The tub gets hot water the fastest - almost in 20-30 seconds. I don't mind the wait for shower so much - it is to be expected with on-demand heater. The most irritating problem is that it does not hold temperature through a short shower. I wonder if the sensors in kitchen and shower fixture are worn out -- they get the most usage. All fixtures are original to the house - which was brand new built - 5 year old.

Both the kitchen and shower have flow restrictions. The tub does not. It could be that if you set the temp to 'warm', you are not using enough 'hot' to keep the on-demand heater active. They have a minimum flow requirement.

Sensors??? You mention sensors in fixtures? What are sensors in fixtures?? Please God please dont tell me they have made plumbing complicated now too, along with cars.

We removed the water flow control piece from the shower head long time ago. Since I posted this, we have checked the water pressure at the tank - it was rather high, about 50; - we set it lower - at 25 -- that was too low. So we set it at 35 and it seems to be working for now.