There are a number of reasons you might want to add an extra SCSI controller in VMware ESXi, such as creating a cluster in Microsoft Windows or database performance. To add an extra SCSI controller in VMware ESXi, simply add an additional hard drive the normal way (Edit Settings -> Add Hardware -> Hard Disk…), then specify that the hard disk uses another SCSI port.
Make sure that the SCSI port you use is at least one number higher than the other SCSI devices. For example, if your other hard drive is on port 0:0, use 1:0 to add another SCSI controller to your VM. Using another port on the same controller will not create a new SCSI controller in ESXi (for example, 0:0 -> 0:1).