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Hi Denise,
I'd like to explain that we cannot change the approve order type when we are trying to add a new approver to the SharePoint 2010 approval workflow (out of box).
We can only select the order type when we are starting the approval workflow, One at a time (Serial) or All at once (Parallel). After that, every time the existing approvers add a new approver to the workflow, the approve order type will inherit the order type how the prior approver was added, we can't make changes to it.
For example, the order type was selected to Serial when A was added as an approver. Now A wants to add B as an approver to the workflow, then the order type of B will be set to Serial.
Given this situation, you may try selecting the approve order type to All at once (Parallel) when starting an workflow.
Furthermore, after one person is added as an approver, he/she can go to Workflow Tasks directly to check the tasks that need to be approved.
Besides, here is a reference that may help you learn more about SharePoint 2010 approval workflow: All about Approval workflows
Regards,
Joanne