Hi Keith
what you're after is the default behaviour of Project :-
if you enter a task with a 40d duration and assign one resource (at 100%)
the duration will (all other things being equal) stay at 40 days, if you ADD
another resource (at 100%), Project will decrease the duration to 20 days.
If this is not happening automatically, check that the task type is Fixed
Units and that Effort Driven is ticked (double click on the task) and that
calculation is set to automatic (tools / options / calculation tab).
cheers
JulieD