Title Says It
Also With Those 2 Other Units
What?

That would be easy, center location a enemy unit(better off 1) and if you killed that unit, make triggers like wait a couple milliseconds that it can't find that enemy unit, then spawn the 2 units.
Or use a queen and do it on terran/zerg ground units or organic.
WEll you could try Krazy's way or you could post your trigger or map and show us the problem. You may have a mistake. You could always check the tutorials

I think what you have in mind is quite impossable.
When you have 1 unit it is easy
When you have a few it is diffucult
When you have alot, like a madness its impossable to make practical.
Now you could create 2 units for a player whenever they receave one death of that unit, then subtract the death. But to figure out where the specific units die when you could have hundreds of them is not something that you can do.