I do double-shifting all the time, but it's usually to downshift half a step (such as front 3 to 2, rear 6 to 7). I haven't seen anything odd in terms of wear and tear.
Bicycle chains are amazingly strong and durable. It'll take much more than shifting both derailleurs at once to damage one.
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Jeff Wills
Comcast nuked my web page. It will return soon..