I do not think there is an easy spell to break up a couple. You could try something to make them hate each other. Just so you know, it can have an impact on your life that you might not like. Just fair warning.
If you insist, I would try something like a reverse binding if that makes sense. Take a block of wood or stone. Glue a picture of each person on opposite sides of the wood, picture side facing out. Light a black candle and drip wax in a circle about halfway between the two pictures, so that it creates a dividing line. While doing so, visualize them drifting apart from each other. You could also write up something to speak as you do so. It does not have to rhyme, but should flow from one line to the next. Make sure you keep the words to your intent. Place it in a container to seal it, then bury it in the earth if you can.
Hope that helps :)