Well there are a lot of great threads on this topic, even for the different types of programs used to do this with.

A simple macro to have the 2nd char follow you would be (and yes keep in mind, you will have to click it in order for your char to follow your main again once follow is broken..)

/follow party1 (if your main char is leader of the party..which it should be)

or

/follow yourmaincharname

likewise targeting would be using the /assist command on your 2nd toon..

there's even a multibox addon (link and name escapes me atm) that will relay a message when follow is broken..so..just have a look around :-)