Although they are compatible i would not mix them in the same bag. You could instead use a Y-port and flow them through the same IV. Potassium chloride is a very dangerous drug and mixing more than 1 med in the same bag is just asking for errors.
I would clarify the order with the physician and the pharmacist, and would not mix in the same bag as my gut reaction. For several reasons, including the proper dose of both are being administered over the correct amount of time and what not.