Yep it is possible to structure your units in that way. You need to select 6 core biomed units in the first year and use the slots which would've been electives if you did a single degree for your commerce units. You can then do that in your second year as well.
However, it is important that when you get closer to the end of the three years that you drop the commerce degree as they won’t let you apply anywhere that doesn't allow early exits if you are only in the 3rd year of a 4 year degree. I know someone who had a problem like this recently and they cut it quite close in terms of dropping the commerce part of their course and applying for postgraduate courses.
So for your last question yes you need to be completing a degree in the same year you apply for postgrad for the most part.
Thanks for the detailed reply, really appreciate it. In your friend's case, I assume they had to drop commerce before the end of 3rd year? Hypothetically, if they didn't get into medicine in that year, what options would they have to reapply?
Also, after dropping the commerce part of their course, does that mean they no longer have an option to pick up the commerce part of their degree if they don't get into the postgraduate course of their choice?
Finally, any idea what the workload would be like with biomed + commerce? I've heard biomed itself is already quite demanding, and I don't want to place too much pressure on myself given that I'll have to spend time in 2nd and 3rd year on the GAMSAT.