You may need to rethink your shower plans. a 1/2" supply is probably marginal, and if you actually run it through 1/2" pex, almost certainly - it would depend on how many gpm the heads require. The ID of pex is smaller than copper (the OD is the same, but the pex wall thickness is considerably bigger). If you ran 3/4" pex, you'd have about the same volume as the 1/2" copper, and even if you extended it in copper, 1/2" might not be big enough. 1/2" pex is probably good for a max of 4-5 gpm. Now, mixing it with some cold would allow that to go up some, since you'd have two independent lines feeding the valve, but it depends on how hot you run your water heater and how much it cools, and how cold the cold is the mix you'd end up with.