The view is much dimmer using the double stack, but boy I found the increase in feature definition well worth it. I had the same problem as RR with getting focus (using a DMK21AS04.AS) so I used one of these adapters
http://www.myastroshop.com.au/produc...asp?id=MAS-278 (won't fit a ToUcam, only a webcam with a C/CS thread) and ground the shoulder down at the thread end. Worked like a charm (after removing the screw on CS fitting).
The trick is to try to get the chip as close to the scope as you can. Some webcams have the chip set a fair way back.
Another way to get around it is to screw the lens of a barlow and screw it into the adapter. You get a slight magnification factor (not 2X) and you will probably be able to get to focus. I could using a generic Celestron 2X omni barlow and a standard ToUcam and adapter.