It's valid when it is in order, double spending cannot occur because of the order of the tokens! You cannot suddenly have a coin or spend it twice, the first one is always valid...
But it could happen that tx1 spends all the funds, which would make tx2 invalid. When you receive the votes and reorder, and realize tx2 is invalid, what do you do ? Broadcast a reorg?!
It's valid when it is in order, double spending cannot occur because of the order of the tokens! You cannot suddenly have a coin or spend it twice, the first one is always valid...
But it could happen that tx1 spends all the funds, which would make tx2 invalid. When you receive the votes and reorder, and realize tx2 is invalid, what do you do ? Broadcast a reorg?!