Hi Marc,
It is more than likely that it is limited to 1 mirrored port on the 2920 switches. I don't have one to check against, but if you are able to assign multiple mirrored ports you would be able to see it listed when you perform "mirror ?" or "mirror-port ?" on the console, this is the output from a 5400zl Chassis (K class firmware) & 3800 series (also K class) which allows 4 seperate mirrored sessions...
SWITCH(config)# mirror
endpoint Remote mirroring destination configuration.
<1-4> Mirror destination number.
and here's the output from a 2620 (RA class firmware) switch that only allows the one...
SWITCH(config)# mirror-port
[ethernet] PORT-NUM Define the mirror port for diagnostic purposes.
So I think you can say that if your WB class firmware gives you the mirror-port option cmd then I thin you are limited to 1 mirror port.
Someone else who uses the 2920 series might well be able to confirm this.
HTH