Duplex rework - PEX size with main water feed to two water heaters

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Chel_in_IL

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My fiance' and I have a duplex that is 100 years old, has an upper and first level apartment with shared laundry in the basement. (We live here, don't rent it out.) I completely reworked the plumbing in my old house with PEX and manifolds, which was fairly easy as everything was accessible from the utility room. The layout I'm dealing with here has me questioning the design. I don't know how old the PEX here is, but we've had several pinhole leaks and have some other frustrating issues that I'll get into, so I think we need to do some rework.

While we inhabit the entire dwelling, we don't want to change the utilities to single in case it is again rented out to separate families in the future.
The current layout is we have a single main water line coming in from the meter in 3/4", branching off to cold in 1/2" and to the two water heaters in 1/2". Both cold and hot are going to the fixtures in a trunk and branch design in the basement. (The original pipes still run in the walls to the first and top level fixtures. We can't easily change that...)

3 issues -
-When the upper floor uses a higher volume of cold water like when flushing the toilet, the first floor loses the flow of cold water and someone taking a shower gets a blast of hot water. (The fact the first level shower has the hot/cold piping to the valve backwards may be contributing to this.)

-Very low volume in the upper floor when the first floor or basement laundry is using a higher volume of water.

-Lower hot volume in upper apartment. I have to divert the shower with cold to get flow to the shower head before I can turn it to hot.

My assumption is that the main water line being reduced to 1/2" before the water heater(s), and also the trunk and branch design in 1/2" to the fixtures may be contributing to these issues. When I redid my last house, I used 3/4" to the water heater, 3/4" to the manifold, then 1/2" to the fixtures, and had no issue with flow between all the fixtures.

So my questions are.. am I correct? And if I am, considering that we have two water heaters, what size PEX should be run to them, which would break off before the water heaters to supply the cold feed to both levels? (For example, 1" from meter to hot/cold branch, then 3/4" to each water heater, and 3/4 out to submanifolds to fixtures on 1/2".) And if I'm not correct.. I'm up for suggestions.
 
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Reach4

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