In our environment, we have never used split networks for production and backup traffic. The sheer overhead of additional network connectivity provides very little benefit. In our case, we’ve had servers with single network NICs and servers with dual, redundant NICs. Adding yet-another NIC, just for backup traffic, would only serve to add more confusion and physical congestion.
In other environments I’ve been engaged in, we used dual physical NIC in redundant configurations, but used VLAN tagging to logically isolate backup traffic and production traffic. This required significantly less network cabling, and it still provided a bit of performance improvement, as our Cisco switches let us mix MTU sizes and leverage Netflow and other features for performance benefit.
Let us know if that helps!