When you book a redemption using BA miles from a HH account then the miles are deducted from each individual's account on a pro-rata basis to the total number of miles in each individual's account that is part of the HH account. So, it doesn't matter how many miles are in each individual's account. What matters is that the aggregate total of miles in all of the induvidual accounts is equal to or greater than the number of miles required for the booking.
As to seeing only one companion voucher, you will only see the companion voucher for the BAEC account you are logged in to at the time.
P.S. welcome to FT Madge Ishen.