I'm not sure whether this will help or not. But I didn't have any problems with the power or HDD's, my main problem was fitting it all in the case.
I did this back in March.
I used:
1 Stock Seagate HDD
1 Maxtor 160gb HDD
I picked up a long IDE cable. Took apart a different cable, to get the connector off of it. Placed the connector at the desired position on the new cable, pressed it on. Now I had a cable with 4 connectors on it (total). (I can't seem to find the IDE cable I made. I would have posted a picture of it.....)
Then I got a Y-adaptor for the power plug. I cut the red wire before it went into the Y-adapter, ran the motherboard side of it to the middle of the switch. I then ran a wire from each open connection of the switch to it's designated HDD. So when I flip the switch, it only gives power to that HDD. This worked perfectly.
Like I said, I had problems with fitting it all and then realized I was doing all this for nothing. The XBOX hadn't been on Live yet, so I just put the 120gb in the XBOX. and was finished.
This is a pic with the switch installed.

This is the back of the switch.

When I decided not to use the dual HDD's I now had a hole in the front of the XBOX, so I put a sticker over it.

This was the whole thing.

I now have a X3cp, so if anyone wants/needs this faceplate. Let me know. It'll come with the switch, and the X3 switchy thingy.