When sending a sign request, there is a "Signing Order" option.
You can can select "All at once" or "One at a time."
All at once:
Everyone can sign the document as soon as the sign request is created.
One at a time:
Only one person can sign the document at a time.
The order of signers is designated by number and position of the signers as they are listed in the request. This order can be changed by dragging signers up or down in the list.