You don't update balances, you enter transactions. Then you derive balances from transactions.
You can even insert them in an unvalidated state, then validate them later. That way if you have two transactions that come one after another, it doesn't matter because you can process them sequentially anyway.
You can even insert them in an unvalidated state, then validate them later. That way if you have two transactions that come one after another, it doesn't matter because you can process them sequentially anyway.