Tried to create a little scenario-without any luck

[Coiler]

For multi-unit airfields, you need:

-Aircraft parking (tarmac, hangars, shelters)
-Runway access points
-Runways themselves
-Munitions storage facilities (ammo pads, bunkers, etc..)

You don't need fuel tanks or other buildings as part of the group, but they're good to have for flavor/extra targets. Once you've put them near each other, group them together and you have an airfield.