I wouldn't bother with pointing jobs to different NIC's for a number of reasons...
1. Gb is good for roughly 80-95MB/s. Most file servers at best average 15-40MB/s. Trend your backups to see where they are at, and who knows you could have 3-5 simultaneous streams running at the same time with no changes at all.
2. Teaming is a much more simpler and elegant solution. Team the NIC's on you server, give it a virtual IP, and now you'll have not quite twice the bandwidth, but the software will automatically load balance the traffic as new jobs start. Usually it takes a round robin approach, but that is fine by me.