I found a way to accomplish this as it currently is:
Controller 1: set its loop mode to "repeat"
Controller 2: set its loop mode to "none" (and optionally its play position to the very end, to illustrate how it works)
Now in Controller 1, toggle Controller 2's loop mode once (to "repeat"), and right after this, "toggle play" it, then immediately toggle its loop mode twice (back to "none")
In Controller 2, "toggle play" Controller 1 (to stop it) at 0.1s AFTER it's looped back to the beginning.
That's all. Since Controller 2's loop mode is set to "normal", it will stop at the end of its run. Running Controller 1 again will cause it to loop once more. You can do this as often as you want, with the same action group! Yay.