OK, so without actually seeing the data you have for those two tiles, here's what I
think is going on there...
So the formation data for those tiles (that is, the data that tells the game which of the loaded graphics to use to display them) comes in the format "cccccccc vhopppcc" where the "c" represents which tile to use (you've definitely played around with that), the "p" represents which palette to use (ditto, I assume), "v" signifies a vertical flip (makes the tile upside-down), "h" signifies a horizontal flip (backwards), and finally "o" is priority. If the "o" bit is clear then the tile will show up behind the character. If it is set, then it will be in front of the character.
So, in order to
set the bit (assuming it's not set already), you add 20 to the second byte of the tile's formation data. To
clear the bit (assuming it
is set), you subtract 20 from the second byte of the tile's formation data.
Take this picture of Edward in (a slightly modified) Fabul. Pay attention to the top of his hat in front of those crumbled floor tiles (and kindly ignore the monks hanging out on the wall)...

I assume those tiles are the ones that you're using. By default, the formation data for the lower two corners of those tiles looks like this:


(to be continued...)