How to handle multiple transactions from the same account(address)?
Confirmation concept is generally useful for the counterparty to be sure that the transaction through which he has received the BTC cannot be reversed. Nothing stops you from creating a new transaction referencing the outpoints (txid and n) of the transaction through which you just spent your 1BTC, if you do not plan on changing the previously broadcasted transaction by say a higher fee. Either both transactions can get mined in the single block, or the second transaction can get mined in block after the first transaction has been mined.