Soft starts work like the name implies, i.e. they allow the A/C to 'soft start' by reducing the in-rush or start up current during A/C start. Start up current may be 30 or more amps, but running amps are in the 12-14 amps range. Both A/C units would have to be retrofitted, unless you only wanted to use one using a 30 amp shore connection. But one A/C will work fine on 30a shore power, so the only reason to use soft starts would be to run two A/C on a 30a circuit.