http://monerotoruzizulg5ttgat2emf4d6fbmiea25detrmmy7erypseyteyd.onion/resources/user-guides/multisig-messaging-system.html
If you issued transfer but really wanted the MMS variant, ignore the written transaction file and simply go on with mms transfer . The MMS does not, or at least not yet, keep track how many signatures a transaction actually has and who signed already and who not yet. Because of this weakness it can include choices that do not make sense, e.g. a choice to send a partially signed transaction to somebody who signed already.