What I do:
If kite has gone down and too light to relaunch (easy, or at all), the board will at first be upwind usually, but pretty fast dolphin downwind, even in the lightest wind as it is the surface waves that gets the board moving.
Sometimes the rider (or yourself) can not retrieve the board before it passes you too far away, and this is BUMMER for three reasons
1. The board can be damaged when hitting something on lower water, or it could get dangerous for others maybe.
2. The rider has to be in the water for a VERY long time if drifting to shore, maybe cold and having to swim as not much flotation in say a shorty or thin summersuit and a harness only.
3. The rider has almost zero chance of relaunching, whereas sitting on the board, it is a lot easier, and even if not, it is relaxing and safe and you can even steer towards where you want to end.
This one is easy, as you ride down to the "lost" board, swap boards and ride his/hers board back up to the rider (or boarddrag up if pretty close) so he/she is "safe" and can sit on the board.
Then just kiteloop down to your own board now, and everything is okay
The other situation is a board that sits on its side so not drifting toward the rider in the water with kite down.
A rare situation in my experience, but today with such light hydrofoils it can happen.
Ride to the board, switch boards and drag yourself with his/hers board to the rider - and then you bodydrag yourself up to your board again - or even better, stay and chat with him/her while you wait for your own board to pedal down to you (as you have of course left it mast down)
If the "lost" board is way to far downwind, you have to take your own board ashore and go get the drifting board out to the rider, of course.
If the board is far upwind, I would either ride or drag the board downwind for a while, stop and drag up to my own board (which is dolphining towards you btw), and ride or drag it downwind past the other board, and so on, so I never get too far away from any of them thus always in sight
Would not try to ride with another foil under my arm - possible if powered, but not safe IMO, but might just be a wuzz in this respect, I know
So one board at a time is my solution, has been and will be.