I'll second Jamba.

With Jamba, you can have the slave mirror the actions of the main, when a window is open.
This means, the slave will take the same flightpath, or choose the same options in a dialogue box.

Basically, you open the npc window on both.
Master toon selects the quest in the window, slave follows suit.
Master selectes dialogue slave does too.

You can set to mirror reward choices (choose the same one) or not.
Or that you need to choose in each window, but then turning in on the master will turn in on both.






Combine this function with two new (3.1 patch) features.

First, on the slave toon, in interface options, mouse section, enable "Click to Move".
Second on the slave toon, go to keybinds and bind something to "Interact with Target".

Slave is following the master.
Master picks a quest npc or flight path guy.
slave assists the master, so they have the same target.
Click interact key.
If out of range, Click to Move, has them move into range.
They then interact with the npc, which opens the dialogue box.
Jamba has the slave mimic the masters choices.