Hi buddy,
I transferred into a double degree, Arts/Law after my first year at Monash University. More information on this can be found on a thread I made in the Monash discussion board. But essentially my weighted average was 80% after one year of study.
Monash seem to give preference to internal transfers, and the 'weighted' average is your average taken across all units you've studied each year.
PM me if you still have any more queries.
EDIT: I forgot to add that the WAM (weighted average mark) required is the same, regardless of what degree you study. That means that someone who scores 80% in Arts is more likely to get into a dual degree of Arts/Law than someone who studies Law and scores 75%.