No.
In Windows 10, the new DX12 *may* be able to do that. They claim we will be able to mix cards and it will work the way we would like it to. But until Windows 10 is released, and the reviewer and testers can get their hands on things out test them out for us, and tell us that things actually do work right, do not plan on it.
Windows 10's release date is set for July 29th. Within a week or two after that, we should hear something on this.