Alan, I don't think the display feature you are asking for is available in the current series of 400/450/550 DualHead cards. While Matrox has superior dual display capability compared to other current vendors, the existing boards only support overlay on a single VGA port. I've had a couple of DualHead boards and I've tried several times to simply get the same overlay output on both ports, to get the capability to use either a CRT monitor or the projector or both at once. I have updated my Matrox drivers several times, but I've never been able to get this to work. It almost does - overlay can be had on either port but not both at once, and switching from primary to secondary is complicated - I've always found the external switchbox easier.
The new triplehead apparently offers overlay on two of the three video outputs. Whether the feature you desire will ever be a part of the released drivers and utilities is the real question. I would have suggested that you submit a request for such a feature in the appropriate Matrox Support Forum, but that "Feature Request" forum has dissappeared just in the past week and the other Support Forums have disclaimers to contact that same rude Sales staff.
I don't want to suggest any cultural bias or gender bias on their part, but a female colleague of mine gets quite polite responses from Matrox Marketing when she calls, as long as she speaks French.
Gary