Multisignature delegated harvesting is an advanced feature that can be complicated if you don’t know what you are doing.
There was an information tab that disapeared but basically it was saying:
Only the cosignatory that initiated the importance transfer can active / deactivate and start / stop harvesting.
This is why it worked only with one cosignatory. Each cosignatory have an unique remote account associated to it.
It is not possible to know what is the remote account of the multisignature account if you are using a cosignatory that have not initiated the importance transfer.
To try fixing your issue you need to log into the cosignatory account that activated the importance transfer, go to “Manage delegated account of multisignature accounts”, type your password, select the account.
Now look at the status:
I - If it says “ACTIVE” it means you have a remote activated.
In this case got to explorer: http://chain.nem.ninja/#/blocks/0, paste your multisig account address to check on the explorer and see if the remote account is corresponding to your cosignatory’s dedicated remote.
In that case you can choose a node and press the start button.
Otherwise you need to find the cosignatory account that has the right remote to start.
II - If it says “INACTIVE” it means you have no remote for your multisig account.
You should be able to send an importance transfer to activate from any of the cosignatory.