LGTM check
You can leverage LGTM check if you want multi-level approval.
The LGTM check requires a "LGTM" comment from project members or owners depending on the setting. A task can only be approved if there is a "LGTM" comment from the designated person (project member or owner).
How to use
The LGTM setting is in the project setting page.
The LGTM check takes effect if the approval policy is "Require manual approval".
The LGTM check fails if there isn't a valid LGTM comment, and therefore the task cannot be approved.
The project member or owner clicks the LGTM button in the comment area to send a "LGTM" comment.
The LGTM check passes if there is a "LGTM" comment from the required persons.