The idea (AIUI) is that it would either be tagged as:
display_name=Derry / Londonderry
name=Derry;Londonderry
Or an alternative proposal:
name=Derry / Londonderry
names=Derry;Londonderry
Amusingly the local vogue appears to be for a tilde, as in Derry~Londonderry. My regex is getting more arcane by the hour.