We can assign Prototype Approvers.
Default State for mocks is In Progress
These approvers then have the ability to Approve a design.
After the first approver approves a design it is locked. Only when all approvers have approved does the prototype status change to approved.

Any approver can change the state back to in progress if there needs to be a change later. All approvers will need to re-approve each time the status is changes back to in progress.