This has been around for a long time. If memory serves me correctly back in the 1980's Air Europe used this system for many years. During the summer months Air Europe would receive a couple of aircraft, I think B734's, from Air Florida.
At the end of the season these borrowed aircraft would be returned to Air Florida. I think these returned aircraft would be accompanied by a couple of Air Europe's. So both airlines would benefit from an expanding and contracting fleet size as their seasonal business demands dictate.