I'm not sure your erratta resolves the primary issue with the spell: namely that it is hard to work in a turn based system. Consider the Attack option. Ok, you and your linked ally are going to attack the same creature. You have initiative so you full attack first. You roll die and your ally rolls a die then you pick which one to use. It's all great until you drop the bad guy on your second attack. Now what? Does your ally have to attack the downed creature in order for you to get the bonus? Ok, let's say the bad guy is still up and now your ally is attacking. Do you both roll again or is he supposed to have written the rolls down so that he knows what he's got?

The mechanic requiring you both to take the same action against the same foe to get any benefit is too fiddly and doesn't play nicely with the turn based system.

If I were writing unofficial erratta, I'd let you roll twice and take the better number whenever you attack a guy your partner attacked in the last round or a guy who is threatened by your partner. For spells, make it work the same but the foe needs to roll twice and take the worst if you cast a spell targeting a foe who was targeted by your partners spell within one round.

That's a more significant rewrite and changes the power of the spell a bit but at least it is clear how it works within the turn based d&d system.