Assuming you use a method other then focus for assists...

You can create a macro per toon (master and slave), which sets the focus.

Could be:
(On Master)
/focus

(On Slaves)
/assist [target=party1target]
/focus
/cleartarget

This is assumes party1 is both the party leader and your current active/master toon.
You will tab or click through the opponents/targets, targeting more then one of them.
That way, they cannot be sure which one you have focused.
The slaves might require the master to have targeted something for a second or so, but will not appear to ever acquire a target themselves.

For your dps macro...
If you go with:
/cast [target=focus] or /castsequence [target=focus]
The slaves/master will be casting at the focus, without ever needing to target them.

The three main assist methods seem to be:
- Focus your active toon, and assist your focus.
- Active toon is promoted to party leader via /script PromoteToLeader("NewLeaderName")
- Some variation of an FTL (Focus free, Target free, Leader free) system using Keyclone Hotstrings. Not sure if HKN or Innerspace can do an FTL system.

With Focus use...
/assist [target=Focus]
/cast Whatever
-or-
/cast [target=FocusTarget]
/cast Whatever

With a Leader system use...
/assist [target=Party1]
/cast Whatever
-or-
/cast [target=party1target]
/cast whatever

Not really sure the syntax on the FTL's