I found what I wanted by editing the virtusertable in /etc/postfix and adding the following below the MAKE MANUAL CHANGES BELOW THIS LINE which ISPConfig puts in:
Tested this and it works great...all email regardless of alias is sent out onto the 2nd domain....and ISPConfig already handles whether the domain is local or external, so didn't have to change anything else except this one change to virtusertable.
Remember to rebuild the virtusertable using
makemap hash virtusertable < virtusertable
Thanks for your help Til.