I'd like to have two Zerglings always be selected and receive orders almost like they were the same unit to the player controlling them.
Though enemies could target the individual Zergling from the pair.

If this is possible through the unit data it would be better than using triggers, so maybe the question belongs there.

Thanks in advance.

Add a conjoined behavior to both units at the same time -> units are linked. You can specify what they share, e.g. selection or life.
In your case, I would use acquire and selection.

For orders, you would need to use triggers.
To avoid infinite loops of orders, use a custom value to mark the ordered unit to ignore the order. Else they would order each other indefinetly.

Gotcha.
Will let you know how it turns out.

It sounds like I don't need to worry about this since shared selection is a built in feature but...
Is there a way to intercept if the player attempts to un-select one unit from the pair (via shift click)?
That is, perform the "pre-action-completion trigger".
I'd like to avoid the "flickering" you would get from "post-action-completion trigger" or the dreadful periodic enforcement trigger.

Thanks for all you help Ahli, you're a gentleman and a scholar.

He can't unselect one of these units. I believe.

Larvae use that when you enable that feature in the game options, I believe, and you can't deselect a single larva then. I think you will be deselecting all, if you try that.