I have checked the RV016 manual and I like this router a lot for its features.
Let me clarify that the group of PCs that you will have a rule to go through WAN1 or WAN2 can't be load balanced as the rules will overwrite the load balancing mode.
- Go to "System Managemen" -> "Multi-WAN"
- Select the WAN you need and click edit.
- on the button you will find "Protocol Bindings"
- for the service select "All Traffic"
- in source IP type the range of IP addresses from - to , if its only one address type the same IP in both text boxes.
- in destination IP type 0.0.0.0 in both text boxes.
- check enable then add
Do this for other WANs
note that If IP addresses that does not match with any rules will be load balanced between all WANs...