Practice Ignition doesn't currently support multiple signatories in a proposal.
However, you can use this workaround:
Create a proposal and send it to the first contact.
Duplicate the proposal and switch all billing types to included service so that the revenue dashboard is not affected.
Best practice is to include a personalized message to state "all pricing has been provided in the original agreement as delivered to (Contact Name)", Or reference the full fees in the service description.
Switch off payments.
Send the second proposal to the second contact