For some tasks (like Search and Rescue and mapping) what you can do with one vehicle, you can probably do faster with multiple vehicles. To test how well it works in practice I tested controlling three Pixhawk powered IRISs at the same time from the Mission Planner using an Antenna Tracker and six 3DR radios (jump here to skip the intro). A transmitter was attached but I didn't use it.
The setup was:
To control the vehicles, I:
Of course, there were some issues (I ran the test 3 times, the video is from the 2nd time). The biggest problem was that there was too much network traffic clogging up the 3DR radio links so sometimes the vehicle wouldn't respond quickly to the actions sent from the mission planner. So in my first test it was hard to arm and take-off before the vehicle auto-disarmed. We think the issue is that I didn't reduce the data rate and the antenna tracker combines all mavlink streams and then resends them out on all channels. This means a lot of bandwidth is being wasted sending, for example, copter's #1's position to copter #2. We probably want some vehicle information (like position) sent to all vehicles but other data (like IMU readings) isn't useful.
Another small issue was the Mission Planner would not let me send new commands until the last command was accepted/rejected by the vehicle. Makes sense in a single vehicle environment, but at times I wanted to submit a command to each vehicle in rapid succession without waiting for the first vehicle's response.
If I was going to do this often, I would change the setup so that there was a single TX/RX for each vehicle to allow faster manual take-over of a single vehicle. I could of course bring all three vehicles down by switching to stabilize and cutting the throttle, or I could switch a single vehicle to Loiter through the MP and then take-over with the RC but that's all a little clunky in an emergency situation.
I might also use RFD900 radios (or re-flash the 3DR radios with the RFD900 firmware) which would remove the need for the antenna tracker completely because it support multi-point communication.
So some more work to do including getting other GCSs like Droid Planner to handle multiple vehicles as well as the MP but still, I was pretty happy that it worked and I wanted others to know that this multi-vehicle style flying is possible.