In GPOADmin you have the ability to configure groups and/or users inside the Approval List in order to allow approvals coming from multiple users.
On the Approvals tab you will find a checkbox telling "Process Approvals in Order".
The issue is that the approval process must be made in order. Disabling/Unchecking this checkbox may render the approvals via email partly not working and only the first approval via email may work while subsequent approvals must be done via the GPOADmin Client/Console.
Having this checkbox enabled/checked, which is the default setting, does not cause any issues and the approvals work as expected.