I did an end-to-end coinb.in test.

1 - I created and funded two wallets A and B

2 - From A I created a 2-2 multisig address with the public keys of A and B.
Address:
3LvaMMABYXamGkH1DQbLfB4pYp8pU3of7T
I saved the redeem script.

3 - From B, I sent money to 3LvaMMABYXamGkH1DQbLfB4pYp8pU3of7T
Transaction:
https://btc.blockr.io/tx/info/230f6e6c3d8bca2686cdfaeda7b24f1f08d657f82cdb82a687a89834e84be52c

4. From A, I created a transaction to send the unspent output of 3 to an external address, loading the inputs from the redeem script (output of 2).

5. From A, I signed the transaction and verified it (signed 1 of 2 OK).

6. From B, I signed the transaction (output of 5) and verified it (signed 2 of 2 OK).

7. From B, I broadcast the transaction (output of 6).

8. Received:
https://blockchain.info/tx/e1e14eb2a56df5812bc4d8b2deea64cf978767517ec38c397ede2c68af6a6762

I will do more tests appropriate to the specific use case that I have in mind. In general I thing coinb.in is great but perhaps too difficult for today's casual users - those who need click and simple one-click solution and are unable to open a new window or copy/paste. However, I am sure that it's easy to adapt the user the interface to specific use cases, hiding complex features and selecting the needed data (e.g. key and transaction to sign) automatically.