to allow to bind to a specific local IP

I have a dual-homed NAS (2 network cards, connected to 2 different uplinks). I want to be able to 
* upload via the network connection that is the fastest
* run 2 simultaneous instances that each use a separate upload link (so load balancing/double speed)

For the last option I still need to make a script that splits up the uploads to be done in 2 parts and then starts 2 S3cmd instances with different --interface parameters