as far as im aware you cant mix and match ATI cards without having a screen or dummy plug on the 2nd card you could always try making your own dummy plug with a DVI to VGA adapter but with the 2 cards being from different series of card its the only way i know of to get it to work the way you want
That's great, thank you! Just knowing it's not me being silly and this being the only known work around is great as I now know it can't really be improved upon.
It may be worth noting though that a real screen can be used temporarily, so you plug it, the card wakes up, and will then stay awake/enabled until the next reboot. This means you can use a real screen and don't have to trick it with some sort of active adapter.