Re: 3/4 water supply with 2 1/2"
I would like to point out that there is no free heat in this system, the water is taking heat produced by the boiler or heat produced by the elements in the water heater.
Also you would need anti-back flow valves and if one source is higher pressure than the other it would simply close the back flow valve and you would only have water from one source or the other. With only ½" feeds you would have a lot of trouble maintaining pressure at the shower if someone turns on the hot at a sink.
Be sure you live your life, because you are a long time dead.-Scottish Proverb