Parallel approval nest sequent approval for SC

Dear All

     Ｉ am using SRM7.02.

　 I can realize parallel approval for SC  as following.

     only after A1 and B1 approved. the process will end or enter next level.

   But , the customer gave me a new challege,  as the following.

The work item is sent to A1 and B1, after A1 approved , A2 would receive the work item.   after B1 approved, B2 would receive the work item.

Only  after both A2 and B2 approved.  the process end or enter next level.

any experts  can give me some suggestion?

thanks in advance

Jesse