As far as i know the idea of having a second focus target was addressed before, ending up with the result that it's not possible.
I wonder though, if a decent workaround could result in a helpful addon. The idea as an example:
A hotkey "saves" your target/mouseover/whatever and displays a unitframe for it, just like a blizz-targetframe. If you choose a different target, this unitframe remains, just like a focus.
It can also be target by spells and so on with a hotkey, making the addon use /target xy at the saved unit.
This would never be 100% reliable, especially in pve with several targets having the same name. In pvp it could be helpfull though. Possible?