multisig tx 1 out of 2 Here is an example of multisig tx

https://blockchain.info/tx/09dd94f2c85262173da87a745a459007bb1eed6eeb6bfa238a0cd91a16cf7790?show_adv=true

the 3rd one output is 1 of 2 keys. 

So the question is what is going to happen if 
both addresses will try to spend that transaction ?
Each private key can send the transaction fully, similar to having a joint account, the other party can cash out the account at any time and so can you.