In workflows it is possible to assign a approval group as the approver.
Then it is possible to set up an approval sequence:
1) First mandatory approver
2) Second mandatory approver
Both approvers must approve in this specific order

or like this:
1) One of the mandatory approvers
1) the other of the mandatory approvers
Both approvers must approve, but in no specific order

A lot of users request a third option:
0) One approver
0) Another approver
but after the "first come - first serve" principal.
When the first approver has approved the workflow, the second approval is removed
So here it is one or the other.