You'll need some form of assist.

The easiest is a focus based approach.
If you're willing to work at it, the strongest is called an FTL Assist.

Anyway, for your question.
Target something with the main/active toon.
Press your assist key, so the slave/non-active windows now also have your target.

In warcraft options (mouse options), enable (check) click to move, probably better for all toons, but the slaves at least.
In warcraft keybinds (targeting functions), set a keybind for interact with target. In the case of both melee toons and non-melee, two keybinds is a good idea so you can IWT with everyone (quests, flight masters, inn keepers etc) and with only the melee (combat).

So you target with the main.
Press your assist, so the slaves get the master's target.
Press your IWT keybind.
Which will have everyone target the master's target and open the quest window.

At this point, anything you click with the active toon will have the slaves mimic your actions.
This works great when everyone is on the same stage of a quest.
But it is actually clicking the same option number.
So if they're on the same stage, then option 3 is option 3 for any of them.

In Jamba you can force each team member to take the same reward, to select their own rewards, to go with what the team picks (if they haven't picked one) but to go with their own choice if they have etc.
This setting will depend on what you want from the team.
An all shaman team might be happy to mimic reward choices, but a Paladin, Priest, Mage, Rogue, Hunter team will want different choices per toon.