Not sure I completely understand the question, but one would generally top off subway tile with a bullnose. Bullnose is also needed at any outside corner (if any) that you turn or at the end of a run if it is open.

If you mean the corner and connecting the two? They sell little corner pieces that tie them in

The subway tile in my kitchen has the matching bullnose tile ran vertically along the side edge and then butts into the contrasting top trim tile. The top trim has right and left edge pieces with radius corners

You can buy the bullnose corners or you can cut 45 degree angles on the bullnose tiles and miter the corners