what do you mean by "the same timeline?" You mean Lifeline?
If so, how would you be able to send two messages exactly at the same time
from one process? This is not really possible, right? There is always kind
of a delay while sending the two subsequent messages.
Parallel does not help in that situation at all. Paralllel simply says that
either Message1 or Message2 can be sent first, but it does not say anything
about the point in time.
You could specify this by using a DurationConstraint pointing from
MessageEnd1 to MessageEnd2 with Interval [0..0]. My interpretation of this
is that there are at most 0 time units between the first and the second
Or you could have a look at Observation, TimeObservation and
DurationObservation. It allows you to involve Observation points in time in
Time- and DurationConstraints.
Same holds true for the receiving MessageEnds of course.