Just to clear it up. If a unit can be credited towards the major of two different field, you can only use that unit to satisfy ONE major. It cannot be shared and contributed to both majors.
Eg. Major A and major B has unit 1 listed as one of the subjects that can be used to fufill the major. Unit 1 can be chosen to contribute towards major A OR major B, not both. So basically my point is that there is no shortcut when choosing 'brother' majors
I thought this was only for second year onwards?
First year subjects can be double counted for minor/major purposes (eg. BIO1011 + BIO1022 can be counted towards two majors for progression or a major and a minor).
Otherwise you're spot on.