My latest follow macro for my ALTs:

/cleartarget
/target <main character>
/focus
/follow
/stopmacro [nomodifier:ctrl,nodead]
/target <secondary character>
/focus
/follow
/stopmacro [nodead]
/target <secondary character 2>
/focus
/follow

The ALT follows the main unless the main is dead or the ctrl key is held down, in which case the ALT follows a <secondary character>, unless he too is dead, in which case the ALT follows <secondary character 2>.

I set focus on whoever is being followed as most all my spells are macros that do /assist focus for targeting.