I interpreted this differently.

when Mrs Madge logs in to her BA account as Mrs Madge, she can see one voucher (hers)
When Mr Madge logs in to his BA account as Mr Madge, he can see one voucher (his)

Even if you are head of the household account you can't see both at the same time but there are two there (compare the voucher numbers when you log in to each account). you just decide who to log in as based on which one you want to use (which expires first etc).

I could be wrong as our's are never around long enough to have two at the same time but that's how I thought it would work.
And as for the miles, as every suggests they'll pro-rata them based on the % of miles you each have from the total pot. so as long as you have enough overall for both trips, you should be fine.