About as easy to run 2 lines as one, same access troubles, PEX is easy
to work with once your in there.

I would at least run all new pex before repairing walls, why do THAT
twice??

Busting into slab MAY do colateral damage to hidden sewer lines etc.

Better to go all overhaead ONCE and be done with it, may be advantage
at resale time too!