shower drain vs tub drain--connected to toilet????

I have a very small master bathroom, floor space is less than 30 square feet. I have a nasty old whirlpool tub that I wanted to eventually take out and put in a double shower stall, which I really don't want to fool around with now because of the expense and time. I did, however, want to put some tile on the floor because there is gross linoleum in there now. (I was looking forward to the challenge, as I have never tiled anything before, and figured it would be an easy beginner's tile project because it's so small).

Anyway, my father in law, who is a retired plumber, told me that I would have to pull up all the tile when I wanted to put the shower in. Something about how the drain would have to be connected over by the toilet? This makes absolutely no sense to me. I've been googling to see if I can find an answer to this puzzle, with no luck. The way I see it, there is a drain there already, for the tub, so why couldn't the shower drain connect to that?

Any advice would be appreciated!