How to limit current on parallel outputs from a power supply?
It is not needed. A device will 'drain' energy from its power source, the power source will not use 'all' of its Amps to connected devices.

That means , with a 5V 20A power source, you can add as many devices from 5V upto a total of 20A (or a bit less to keep some margin).

You will get in problems, if you connect devices totaling more than 2A to a 5V 2A power source.

So you can better keep the 20A power source.