If by "main db" you mean that only 1 db is connected by tails to the supply and db2 is 'submained' off this board via an MCB, then the bonding must go back to db1 if you're using the earth bar inside as your MET.
If both boards are connected to the supply by tails and both have the correct MEC then you could take the bonding back to either.
In either case the MET could be external to the db.
The bonding only needs to be done once and must be connected back to the MET (is this what you were getting at?)
