You can make an extra bar on the Paly and DPS. Leave it empty on the Paly and fill it with macros/spells on the slave (say, binds F1 to F12) and do not pass the rest of the normal keybinds so you can use the Paly without using the DPS.

That way you can keep the Paly as the main and heal and do DPS with the slave pseudo-independently.