I would say you need to do this yourself in the form and the workflow and won't be able to use the out of the box delegation functionality.
I think to approach this you could have a field on your approval form that is a people picker, limited to members of a specific group. if completed by first approver then you know needs to be deleted. you can check in workflow if "delegated to" field has a value and conditionally assign task based on that.
Does that make sense?
thank you for your answer. Unfortunately i only can use InfoPath and there is no possibility to add a field for an people picker with the required functionality.