Well, let's see...34+75 is 109, now run that through maybe 20-30' of pipe or more, and you'd be running straight out all hot water that is likely at least a few degrees less than that 109 ...maybe a lot. You might want to set the shower temp where you like it and run it into a large glass and then stick a thermometer in it to see what temp you really like. My guess is it may be a bit more than 109-degrees if you like a hot shower. I don't think you'll like either one if you like a hot shower...and, if anyone opens another fixture - say to wash their hands, the temperature will either drop OR it will decrease the flow (can you say anemic shower?) to try to maintain the desired outlet temperature. Is there a way to make this work? Yes, even if you want to stay with tankless...put two in series and you should be able to not only take two showers, but maybe run the dishwasher or run a load of clothes at the same time...it won't be cheap. Easier to use a tank, though, and probably cheaper in the long run.
As the thing ages, you need to delime it or the temperature rise goes down (the minerals act as insulators). how often you have to do that would depend on your water and use.