Well i use this (found it on the internet somewhere cant remember)
Code:
/console Sound_EnableSFX 0
/focus [@mouseover]
/cast [@target] soul swap
/cast [@focus] soul swap
/script UIErrorsFrame:Clear()
/console Sound_EnableSFX 1
/clearfocus [@focus,dead]
It works like this:
You target your main target while applying all DoTs you then move your mouse over the other Target you want and while you have your mouse over him you spam you Macro Key.
This will Focus you secondary target , Inhale from Main target and swap to your MouseOver target while you remain on the Main target DPS'ing.
When secondary target dies it will clear Focus
Hope it helps