I am suggesting that there's a tab inside of each station that says "mutual aid". In this tab, it will look just like the units tab, but maybe only allow units that are appropriate for that station, but any vehicle of the respective categories of agencies can do mutual aid and replace that unit temporarily. When the units that were busy finish up their call and return to the station, the mutual aid unit/s will automatically return back to their station.
This feature can add a lot of realism to the game. In my area at least, all agencies work together in helping each other out when they are handling large incidents. For example, my city had a 2nd alarm structure fire. There were about 4 empty stations. Three of our neighboring departments stayed at a station in the city and filled in spots for any further calls.
