Purchase order Parallel Approvals

[Buddy]

Dear All,

Any one of you let me know how to do acheive Parallel approvals for Purchase Orders.
Ex: Mr.X is having a individual limit of 10,000 USD
and Mr.Y is having also having individual limit of 10,000 USD.

But if the PO value is more than 10,000 USD and upto 50,000 both X and Y should approve the PO. How to acheive this scenario.

Urgent as it is the requirement at client place.

Immediate response is highly appriciated.

Regards,