I'm still pretty confused about this.
Somehow, you think a whisper is a very transient thing. That's not my
experience. Usually, the coach opens a whisper function and coaches
the agent throughout the transaction. It's not a push to talk, it
really is more stateful.
Of course, many sidebars are one shot side comments from one participant
to another.
The thing is, you don't know the user's intent; you don't know if they
intend to do a one shot, or intend to leave the channel open, unless
you give them two UI functions. Since that would seem to be equally
applicable to whispers and sidebars,