Compare Two Users: Can we use the user from the "user reference" field?

I'm setting up an e-mail rule. For a light project management system.

We have content types called "actions." I'd like the system to e-mail the person the action is assigned to whenever there has been a comment on the action. A "someone else has commented on your action" notification.

However, we ought to check that the user making the comment is not the user the action is assigned to. No need to send a "YOU have commented on your action" notification.

Assignments are tracked by a user_reference cck field. The easy way to do this check is to use the "Compare two users" condition. User 1 = User submitting the comment. User 2 = the user in the user_reference field. Negate the condition so that the e-mail will send if the two users are different (if the commenting user is different that the user the action is assigned to)

But i don't see an option to use the user_reference with the "comapre two users" condition.

Is this possible? Any help would be appreciated. Thanks.