Again, you can already do that. You need to make sure the Destroyers are in the right place at the right time (in other words, comms officer needs to be working to do his job). From the example above, just order DE2 to "Defend Artemis". The whole formation should move to you and form on your position. If you are stationary (why?! get moving!) they will circle you. When you move, make sure you keep at about 50 - 75% impulse and they will follow behind you. As you attack a fleet, they will fire upon whichever ship is firing at you. If you want them to break away, simple order DE2 to "Attack Nearest Enemy" and the small fleet will do so. Order the rest while in combat to do the same and they will provide you effective support.
A couple of things to note. Make sure you are completely aware of the position of the allied fleet at all times. I have been on a ship where the Captain conducted a "bombing run", mining the allies as well as the enemies. Your allies will not take evasive action to avoid your weapons fire, so be careful using Nukes and ECMs (you can't hit them with beams). The allies move a lot slower than you. They will not move at warp, but instead move at a speed of ~50% to 75% impulse. You will need to take this into account when planning your attack run, and helm will have to watch both the allied fleet as well as the enemy fleet as the distance is closed. You also need to make sure they are in the right place if you want to use them. Waiting for them to cross half the sector to get in to position can waste valuable time for the stations you are supposed to be defending.
Sorry about talking tactics when this thread is meant to be about development.