Ability to split approval proxy by transaction type?

If a manager is planning to be out of office they navigate to the My System Profile page and set an alternate user to have their approvals route to. If the manager approves both vouchers and requisitions and wants to separate those responsibilities to two different individuals during their absence, how can they set it up so they can assign one alternate user to receive their requisition approvals and a different alternate user to receive their voucher approvals?