The creation of a Household Account doesn't involve any kind of 'transfer' or even amalgamation.
It merely constitutes an aggregation of individual accounts, from which redemptions can be made with the debits from each account reflecting the proprtion of the aggregate balance held by each member.
So, let's say you have 100,000 Avios in the account, and your father has 50,000. You make a redeption totalling 66,000 and the debit will come proportionally from each account; 44,000 from yours and 22,000 from his.
Note that if you wish to make a redemption (other than for the second person on a 2-4-1 companion voucher) you can only do so for members of the HHA. If you are not a member of an HHA, then you can make redemptions on behalf of anyone you so choose.