Trunking or bonding your NIC's would be ideal if possible.
Alternatively, within your job properties, you can select under network/security which NIC the backup traffic will go through. Typically, if you are going to specify a NIC, it's usually a dedicated VLAN or network just for backup traffic, as well as using things like jumbo frames. Not to mention, you need to be backing up to fast disk to take advantage of the bandwidth with dedicated NIC's.