On Sun, 2 Sep 2012 19:10:38 -0700 (PDT), wrote:

I had a porcelain tile crack in the kitchen. In the process of removing the tile, another popped.

Now I have 2 tiles I need to put back. The back of the tiles are both free of any mortar.

My question is regarding putting the 2 tiles back in. I have the new premixed mortar I purchased. Do I need to scrape any more old mortar off the subfloor?
This is what it currently looks like:

http://s1163.photobucket.com/albums/q548/cegarbage/

Is there anything else I can do to ensure the tile doesn't pop up?

You don't say if you have scraped the floor. Scrap the concrete good
and vacuum any dust off.

YMMV