In order to divide your scans among several scanner appliances, the first thing you need to do is assign specific network segments to your scanner appliances. Please follow this article to learn how:
How do I restrict Scanner Appliance to a certain network segment?
Once you have assigned network segments to your different groups of scanner appliances, the group will work as a cluster when performing a scan. The appliance that is the best fit (lowest utilization, least scan jobs etc.) will be assigned the scan and execute it.