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I'd cross off Ernest because it has the same old clunky feel as Arnold if it were a first name, which obviously you can't get around having in there. So putting the two together feels like overkill.

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Gideon is a lot more fresh and "modern" feeling, similar to Theodora, so I'd go for that of the two. I don't think it would get shortened, I've never known one who went by anything but Gideon. Plus it's not really that hard to correct people if they did try to shorten it without asking.