How easy is it for Metrolink Trams to couple and uncouple? As a short term solution, could they get the Bury to Altrincham services (as they are normally doubles) to uncouple at Cornbrook with 1 portion to Alt and another to Trafford Centre.
This does use the 'shuttle' which has been mentioned above but also ensures the doubles to Bury are kept and if all Bury-Alt trams are doubles, then it would use no extra trams.
Have you ever stood on Cornbrook station, noting the frequency at which trams pass through there?
And good luck trying to 'persuade' Altrincham commuters to leave the part of the tram being detached.