Bleh, that's what I get for not seeing the FF4 Editor have some battle scripts not show up properly..

Well, the explaination is for FF4A, but it applies to both the SNES+GBA ports anyhow.
There are better examples though, such as the Fiend monster (Land of Summoned Monsters) that casts Charm as a response to anything.. only one Charm is used in retaliation.
Then, there's only a solo counter from a dual Behemoth setup when you use a Summon.. which has yet to be properly explained. If you use a target them with Meteor or use any spell and target them both, both Behemoths counter.
A setup that does properly counter... 2 Sandmoths and 2 Larva (Kaipo monster).. if you cast magic on all of them, both Larva properly counter with Psych/Osmose. Same thing happens with the TinyMages.
It seems to be specific to the type of counter...
Stuff that has to do with the HP/damage counter probably holds the value of the damage briefly.. so you only see one counter.
Stuff that has to do with the magic counter doesn't.. so all monsters affected will heal.
Let's goto the Cell monsters (those that look like the Virus/Bio spell).
If you cast a spell that hurts a pair of WaterBugs (but doesn't kill them obviously, these monsters are from the the area you find Tellah initially), they both heal.
If you cast a spell that hurts a pair of MoonCells, only of them goes healing.

My Fiend monster recollection is awful, it responds to damaging attacks instead... oh well.