Having both played multi and played against multi in ACBL events, I completely agree with Andy - you can't wait for the opponents to open 2
♦, then look at your hand and decide what defense you are using. In theory, you need to agree before the round what defense you are going to use (whether your own or one of the ACBL's). In practice, when multi was allowed in pair events, I always told the opponents they could wait until we opened multi to decide which defense to use. That saved a bunch of time and in my experience it didn't hurt us. Now that multi is limited to events with longer rounds, I think that it is normal for the opponents to decide in advance what defense they are using - I've even (once) had an opponent ask for my written defense instead of either ACBL one, and I gave it to them
.
My least favorite experience playing multi was back in the booklet days when one opponent thought they were playing one defense and the other thought they were playing the other one (behind screens) and things got horribly tangled. As a result, I print the two defenses on different sheets of paper and give them only the one they say they want to play.
As for the "loop" issue, I remember this coming up in the Bermuda Bowl in Jamaica - that was a long time ago and I can't remember for certain what the opening bid was, but I have a vague recollection that it was either a strong pass or a fert, where the people using the method wanted to change the meaning depending on the defense. I know that eventually someone figured out that they couldn't do that, but I'm afraid that's as far as my memory goes!
Jan Martel, who should probably state that she is not speaking on behalf of the USBF, the ACBL, the WBF Systems Committee, or any member of any Systems Committee or Laws Commission.