That's a good question, and I also look forward to a response from someone who has tried that.
I've been doing TONS of reading, as of late, including information about the A/V Options for the Xbox 360, in preparation for some mods I plan to do later this week.
From what I understand, you can only have one Type of video output at a time.
I think this is because of the way the AV Port has set up the "Cable ID" pins.
In order for the 360 to recognize the kind of AV cable you have connected, part of the cable grounds certain pins to other pins.
I think that the only exception to this might be the ability to output both S-Video and Composite (Single Yellow Plug) both at the same time.
There may be other workable combinations, or it may just be that only one Video Output Type at a time is allowed.
I haven't been able to test my knowledge first hand, though.
At least, not yet.
I'm still waiting on some Xboxes, that I had ordered off ebay, to be delivered.
Hopefully, this next batch won't all RROD on me on Day One
, like the last batch I ordered did.
To be fair, I did get a full refund, sans shipping them back.
This time I ordered 3 of them, from 3 different sellers.
Surely, at least one of them will work...