BPMN21 — Event-based gateways in choreography should be exclusive

An event-based gateway implies a message is being waited for, but
choreographies can't receive messages, they have no central controller.
One of the participants will have an event-based gateway internally, but
the other will have a exclusive gateway. The choreography can use an
exclusive gateway with no conditions, with the semantics is that exactly
one of the following messages will be sent.

Comments:
From: conrad.bock created: Mon, 13 Jul 2009 15:53:49 -0500 (CDT)
Email from Frank about conditions on exclusive gateways in choreographies:

Hello Conrad,

In my opinion there is no need to even have data as decision criteria.
There may be a button, that a user presses (request xy..). Of course you
can discuss, that the fact, that a button has been pressed is in itself
data in the system. But it is a different kind of.

So in the case of two senders there are two users and two buttons.
Whoever presses first sends first.
Whoever sends second is in a different branch of the choreography.
That's how I understand it.