Are xp's that important for you? I mean, a great post would get you what, 100 xp's? So you would end up "cheating" your co-author of 50 xp's. That's about a week worth, if you are mildly active. You made that much yesterday! Nothing to loose sleep about really.
If you feel that bad just spend your next 50 or 100 votes to ++ his other valuable posts. I am sure you will find plenty of opportunities to make up for the lost bounty! Or let the co-author post the node and feel like you've done your Good Deed for the day (plus you've got the xp's for this preliminary node also!).
In short I don't think xp's are valuable enough and are attributed fairly enouh to design special tricks to ensure that they are divided properly in such a specific case. If you provide valuable input to the Monastery you will be recognized, whether you have 50 more xp's or not. And don't sweat the xp's, you will become a Saint later this month or early in June anyway.
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