The idea behind using a second NIC for backup is that the backup is done over a seperate network so that the normal network traffic and the backup traffic do not clash with each other. If you are going to use one subnet, presumably the one where there is your normal network traffic is, then there is no point in using a second NIC.
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.