The way relationships are setup is a simple LeftNodeID, RightNodeID, RelationshipNameID columns. Now as I look at the database, it looks as if there is a RelationshipOrder column too but I don't beleive it's hooked up. So you could do some modification for it I guess but I'm not sure how well it would work. You have the ability to display pages based on the left or right node id and you only have one order set. So if you're looking at the left node id and ordering by the RelationshipOrder, this works. When you go to get the same pages based off of the right node id, the order will be the same as if it were on the left node. Not really ideal, you'd need a left and right order by field.