LoadRunner is agnostic to the number of NICs in a host. So long as you have one it is happy, hence the paucity of information on what if more than one?
Refer to your operating system information on setting up two NICs in a host. Refer to the 'ROUTE' command at the command prompt for information on explicitly defining routes for the transmission and receipt of information to/from a given host. It will be essential to force your routes as your ethernet is likely to be faster than your RF and therefor preferred for routing to your application host under test.
From LoadRunner, your target IP address for the load generator will be your test control network NIC. For your scripts the target will be the IP address/hostname which is directed across the RF link