You could try two keymaps.

Keymap A
Step 1 - Send Interact to every client.
Step 2 - Send the DPS macro to every client.

Keymap B
Step 1 - As above, for all clients except 1.
Step 2 - As above, for all clients except 1.

Or instead of Keymap B, go with Keymap C...

Keymap C
Step 1 - Deactivate Keymap A on Paladin 1.
Step 2 - Activate Keymap A on Paladin 1.

You could have multiple Keymap C's for a bit more control of your team.

A lot of options.