Offer Approvers can approve out of order

Summary

Offer emails get triggered once an approver has approved, then the next approver gets an email. However, any approver can go in at any time and approve via their Task List.

Content

We have offer approvers set up in a specific order for approvals, and the email notifications get sent out this way.  However, an approver can go in at any time and approve (out of sequence) the offer on their Task List from their home page.  How can we prevent this or have it not show up on their task list until the prior approver has completed their selection?