He didn't "spend" them, he just broadcast the transaction. A few of you seem to be misreading this. You can broadcast anything you want; it just won't confirm until it is actually able to be sent. In this case, the transaction was broadcast before he was able to actually send it. As soon as all the inputs were confirmed, the transaction became legal and passed through the network.
But how can I do it again? Multbit won't normally allow me to broadcast those transactions, it just simply says Not Enough Balance.
May be you put a large enough fee on the child transaction to make miners want to confirm both parent and child transactions in the same block. How much fee did you pay?
Just the standard 0.0001 (~4 cents) for both the transactions.
