So, right now you have this?
1. inbound wideip named www.sample.com with two Virtual Servers 1) 100.100.100.100 and 2) 4.4.4.4?
Have you delegated DNS resolution to Link Controller yet?
1. Under Link Controller > Listeners, configure a Virtual IP on each ISP network.
2. From your DNS servers, delegate resolution to Link Controller.
If you were using BIND, for instance, you'd have records like this in your sample.com zone.
listener1 IN A 100.100.100.2
listener2 IN A 4.4.4.2
www IN NS listener1
IN NS listener2
Admin guide covers that stuff fairly well.