Parallelism of duplicate - only using 2 channels of the 4 allocated

Hi,

I'm using this script to duplicate a DB from the prod RMAN backup to the UAT environment (a 4 CPU server):

RMAN> run {

2>   allocate auxiliary channel aux1 device type disk;

3>   allocate auxiliary channel aux2 device type disk;

4>   allocate auxiliary channel aux3 device type disk;

5>   allocate auxiliary channel aux4 device type disk;

  duplicate database 'BLAH' DBID 123456789 to 'BLAH_UAT' backup location '/rman/backups/oracle/BLAH/rman' OPEN RESTRICTED;

}

It definitely allocates all 4 channels:

allocated channel: aux1

channel aux1: SID=584 device type=DISK

allocated channel: aux2

channel aux2: SID=4 device type=DISK

allocated channel: aux3

channel aux3: SID=198 device type=DISK