You could practically open two separate bitcoin clients (such as MultiBit and Bitcoin-spending is a core issue of a decentralized payment networkQT). Unfortunately broadcasting transactions isn't enough to keepPrepare a consensus of the state oftransaction that is using the networksame input. Even if, as you say, transactions reach payees immediately, there's still that tiny amount of timeAs for a dishonest user to broadcast a double-spend and effectively get their money back. A dishonest person could also have a multitude of bitcoin clients and computers in different geographical locations working to propagateoutput one would use the payee's address, while the other client would use your address (you're double-spend spending right after the service was bought?).
This problem is fixed withAt this point you'd just have to send the proof-of-work (aka mining). The state oftransactions at the entire network is kept bysame time and hope your payee has gotten the blockchain where new blockscorrect transaction instead of transactions are added every 10 minutesthe double spend. When blocks are added on top ofI doubt there's a client that automates this procedure.
You could also make this happen with a higher success rate if you send the block where yourvalid transaction was putdirectly to the payee's client, while you send the harder it becomes for someone to double-spend it, as it would require them to redo all the work fromnode of a pool. This would increase the chances that your double spend will actually make it inside a block untilbefore the latest blockother.
So for someoneOf course to actually try a double-spend theydo this would need access to a quite large amount of computational power. And even if they were ablerequire you to find a single block, most payees will wait at least for two confirmationsget your hands wet with lower level bitcoin clients such as (two blocks)sx or more,directly bitcoind. Actually I'm afraid you'd have to program this functionality yourself studying the protocol and making it extremely unlikely for someonesure you know how to accomplish a double-spendbroadcast to specific nodes instead of the entire network.