I work at Spreedly and this is essentially what we do. Store credit cards with us (we're PCI Level 1 Compliant) then decide which payment processor you want to send cards to for transactions. We don't provide automatic routing so you'd need to decide which processor to use on your end but you could easily charge the same card against two different payment gateways if you wanted to.

My company, Invoiced, supports routing payments across different processors/merchant accounts. There are many interesting use cases for it within the billing space beyond just cutting transaction costs.

One common use case we see is trust accounting for lawyers. The gist of it is that trust and operating funds are stored in separate bank accounts, depending on the context of the bill. It’s a legal requirement that can result in disbarment if not followed properly.

Another scenario we run into is with insurance billing where policy payments need to go to different bank accounts or even be handled by a different processor, depending on the policy.

I know of other software systems that can do payment routing as well.

There definitely is not. Good idea though, however the margins would be razor thin. Possibly too thin.