AC voltage regulator in parallel
Most common AC 'regulators' use triacs and control power (it's not really a voltage regulator) by turning on the switch later and later in the AC cycle, a later turn on means less power (it's duty cycle control really), I don't see any major issues paralleling them, although unless they are well synchronised, one will take full power for a brief instant before the second switches on as well, although if the peak current ratings of most triacs and SCRs is anything to go by, that's unlikely to be an issue (10x current handling for one half cycle is quite common)