I have a crazy theory. Nature created Siphoners to stop the merge due to the merge being againts Nature.
The merge creates a huge imbalance of power. One witch with the power of two.
The merge between Kai and Luke was actually a Nature's way in mantaining a balance. Kai (a Siphoner) won the merge and killed his brother Luke (a Witch). So he didn't have a power of two witches, only one+ his Siphoning.
But to end the merge Nature needed two Siphoners.
And when Lizzie and Josie will be 22 they will have to do the merge. They'll hold each others hands and they will be saying the spell ‘Sanguinem desimilus! Sanguinem generis fiantus!’. But nothing will happen at all. None of them will die because there is no magic to merge and create an imbalance of power.