Hi Jeanie,
I hope I understand your question correctly. Do you mean when creating a change set from Source Control of web client, both a comment and a work item association are required?
For that case, you can add the precondition "Require Work Items and Comments" for the operation "Deliver (server)" in the operation behavior. This setting takes effect for delivering using SCM command line and web client.

Not sure if this is the same thing, but we have something similar.  This occurs when users first do a check-in without delivering.  The system does not prompt or force you to enter a comment or associate a work item.  If the user later does a Deliver, the system does not prompt for a comment or work item, it just throws the error that a comment and work item are required.  To get around this, we tell everyone to use the Check-in/Deliver option.  When you do them together, the system prompts the user for a comment and a work item, therefore, no error is encountered.  Hope that helps.