NU_Send_To can not be told which device to use. It will go to the routing table and get the first available device for its subnet. So, what you can do, is change the routes in the routing table. Use NU_Add_Route to change the direct routing. There are 3 types of routes; here we are covering only "host routes." This will assign an IP address destination to a specific device IP address.
Set ip_dest to the IP address that you are sending to, set the mask to 255.255.255.255 (this is a host route) and the gw_addr is the IP address of the device that you want to specify. Call NU_Add_Route with the above parameters to modify the routing table.
The effect is that any packet or datagram sent to ip_dest will be automatically routed through gw_addr, the select ethernet device.