I can understand how reading the SRD might potentially be confusing on this since it has a run on sentence, but the PHB seems more clear with nearly identical, but segregated sentences.
"Your own oak cudgel or unshod quarterstaff becomes a weapon with a +1 enchantment bonus to attack and damage rolls."
Note: Nothing is mentioned about the properties of the club (i.e. cudgel as mentioned as a club in the target portion) or quarterstaff changing except the enchantment bonuses.
"It deals 1D10 points of damage (+1 point for the enchantment bonus) when you wield it."
It merely states what the damage due to the spell will be. It says nothing about how you wield it and since a quarterstaff is a double weapon, you can wield a quarterstaff as a double weapon. Each attack, regardless of from which end, will deal D10+1 damage because sometimes you are wielding the one end (fighting two weapon with a double weapon), sometimes you are wielding the other end.
Nothing in the spell description whatsoever indicates that the properties of the weapon are changed in any way with the exception of the changed damage and the bonus to hit. The lengths are the same. The weights are the same. You can use the weapons just like always (i.e. still skilled in using it).
Caliban said:
I personally see nothing inherently unbalancing to using two shillellagh spells to create a 1d10/1d10 weapon, considering that you still have to spend two feats to use it effectively.
However, everytime I have allowed that interpretation certain individuals have claimed that the shillelagh spell would affect both ends with one casting, since a double weapon is still "a weapon" and spell affects "a weapon".
And they are correct.
Caliban said:
So I decided on the stricter interpretation, to settle the issue.
In any case, you wouldn't be able to cast shillelagh on the same quarterstaff twice, because it can only be cast on a non-magical weapon and the first casting of the spell makes the quarterstaff a magical weapon.
You seem confused. Either it affects the entire weapon, or it does not. You seem to want to have both, but only your way: it affects the entire weapon so that you cannot cast it twice, but it does not affect the entire weapon so that both ends are not affected.
Even your own words indicate that this seems like a personal problem you have with the spell, so you house ruled it.
Your entire argument boils down to it not explicitly specifying D10/D10. That's so weak, it's almost non-existant.