Tile on Tile adhesive and prep

The best way to remove the old tile but if it's not possible so u must scratch them with grandeur as much as you can and clean it very well than prime it wait for the primer to dry than u can tile it and if it's to difficult to grand it because of the dust just clean it very will and prime it it's should be fine Thanks

The very best way is always to remove old tlies first, latex to level floor and then lay new tiles. However if this isn't possible you first need to ensure the old tiles are sound ie. Adhered properly to the floor as if any are loose this will affect the new floor that you put on top. You can as suggested use a grinder to create a key for adhesion but this is messy and time consuming. Make sure the tiles are clean and dry then apply a primer. When this is dry you can lay your new tiles.

Ensure old tiles are firm and not lipping at all. Clean tiles thoroughly, scratch down old tiles and cleaned again. Lay high quality primer over tiles and wait for this to dry. Then tile over with tile to tile adhesive.

Fine to tile on top of floor tiles as long as they are sound. Check for any hollows and if there are any (usually on the corners of the tiles) knock them out and fill with sand and cement or fast set tile adhesive. Scratch up floor to create a key and prime with acrylic primer. Allow primer to dry before starting tiling.