How does Stellar multi-signing happen practically?
The coordination of the tx envelope before it has the right number of signers happens by some other channel. If you and your friend both need to sign for any tx from an account, you would sign it and then email or IM the tx_blob to your friend. Your friend would then add their signature to the tx_blob and submit. 

If your enemy signs a tx you can simply remove your enemy's signature before submitting it to the network.  

If someone doesn't sign then the server you submit the tx to will reject it immediately.

Source: Multisig and Simple Contracts