Dave

I really don't see the need for a slurry coat when tiling on top of tiles . A flexible adhesive will stick to the tiles no problem.....but if you would prefer to use a slurry coat, then use bal bond sbr mixed with Portland cement.. or similar and brush this on too cleaned tiles ......but i think its a waste of time and money...but its up to you ..can only advise you the best way i think.......but if a sales man Say's other wise then he must be right...lol lol lol lol........dave....