How to ensure that purchase orders have two levels of approvers?

RT.

I`ve added both position A ,position B into PO approve hierarchy,

the level of A is higher than of B,and both of them are higher than the buyer who submitted the purchase order,

I wonder how to ensure that when the buyer submit the document to B for approval,

if B has approved it,then it will be submitted to A for approval immediately. which implies two levels of specified approvers.

thanks for your help.