Why did Arwen do that??

[Amarie of the Vanyar]

Saucepan Man, I was literally reading the word 'toghether' in the last line in X Phial quote:

This doom she chose, forsaking the Blessed Realm and putting aside all claim to kinship with those that dwell there; that thus whatever grief might lie in wait, the fates of Beren and Luthien might be joined, and their paths lead together beyond the confines of the world.

Of course you are right that it can only mean that she became mortal and that I am likely to be over-interpreting it; but I think that the fact that she becomes mortal is said in the previous sentence the fates of Beren and Luthien might be joined; and that the second sentence their paths lead together beyond the confines of the world means that they are to die almost at the same time.