Check out the FAQ at Cubeowner
You can get dual VGA with a Promax card or a 7500 (i think) or an 8500 (if you really like to take risks and have the money to repair it in case it fails) but dual VGA is all you're going to get without some serious mods.🙂
You can also put in a GeForce 2MX TwinView card - it's cooler than the faster cards, and gives you an ADC connector and a VGA connector.
Other cards that fit the Cube are the GeForce 3 and the Radeon 7500 - I believe that the 7500 Mac Edition does have dual monitor support, unlike the GF3 (which is better than a GF4MX in many cases - and is why they are rarely seen on eBay...). Just to be sure, though, check out www.cube-zone.com for a complete list of viable cards for your new arrival...
I had a Cube with the stock video card and it had an ADC out and a VGA out. I had 2 monitors going at once with no problem. Are you trying to use 2 ADCs at once? What connections do you see?
that's strange. are you sure you are in extended desktop mode? did you check your settings? these may be silly questions. i never had a problem with a 15" Apple LCD and a 15" KDS, either with extended desktop or mirroring. i'm afraid i can't help you beyond the basics.