English muffins have a unique texture, with little air pockets and a perfect crunch, and they even come pre-sliced. Using a knife to cut one will simply ruin the texture you crave.
To preserve this unique muffin's makeup, poke its perimeter with a fork until it splits. This way, those inner ridges, nooks, and crannies are preserved.
Another option is to pull apart the two halves of the muffin by hand. Do this by gently squeezing the sides of the bread all the way around until it falls open.