We have these questions from time to time - companies have a strict structure how time off is approved by a chain of multiple employees until something can be signed off.
While this does make sense, we've always been encouraging our customers to think from a different perspective - how to make things simpler & speed them up. In practice, when a single employee is approving a time off, it happens much faster & in today's environment speed matters.
There are already lots of edge cases which can happen - employee can edit or cancel time off request before it's approved, change replacement person, etc. Having a chain of multiple approvers would dramatically increase the time to approve a simple time off request.
We've always been advocating simple, clear rules instead of complexity & so far this has worked perfectly well for every one of our customers.
Tip: you can create a Recipe to send a notification to specific employees when their colleague or team member time offs are approved.