Replace one Voltage Regulator by two coexisting regulators (NOT in "parallel")
...I need 5 amps at 3.3V, is there any flaw with just using 2 regulators with 2 x 2.5A that are NOT in parallel, but only with the same GND connection supplying different 3V3 regions on the target PCB?

Yes, that is possible. But don't "see" it as 2 regulators supplying 5 A bit see it as 2 regulators each supplying up to 2.5 A, because that's what it is. As long as you do not exceed the 2.5 A per supply domain you will be fine. This is what is done on many PC motherboards. Count the number of inductors near the CPU, each one is supplying a different supply domain.

If you keep the supply voltages roughly the same value (less than 100 mV difference) then I would expect no problems on the domain crossings.

On a PC motherboard the domains will have different voltages and then levelshifters are needed, most of these are present on the CPU but some can be external as well.