Why do you always want to BCC your email to someone?
There's not a way to do this automatically built into Entourage. You couldn't set up a rule to automatically do it, but it would probably work to set up a rule so that anything that goes into your Sent mail folder would automatically get forwarded to another address. There might be a way to AppleScript this, but such scripting is way over my head. You could try asking the Entourage experts at the Entourage product forums to see if one of them could figure out a way to make it happen.