I have replaced two regular switches with two PD-6WCL-WH dimmers. However, there were only 3 black wires in my wall box, with a jumper across the two old switches. I've tried putting one black wire from each dimmer into a wire nut with the black wire from the box that was jumped across both. The problem I'm having is: light 1 works fine but it is also dimming / shutting off light 2. Light 2 works fine but only if light 1 is on full brightness. Clearly light 1 is affecting light 2. What have I done wrong?