God no, I hope they never collaborate. Imagine if they both sang together, fans of one side would go 'OMFG THAT OTHER ***** RUINED THE SONG" or something similarly idiotic.
Besides, no matter how much I try, combining Ayu and Utada's vocals in my head never works. They'll be too noisy. Utada's ocassionally shrill voice coupled with Ayu's ocassionally old voice. Never.
It'd be nice if they composed and wrote for each other though.
And do the background vocals for each other. That'd be so sweet.