The following applies to the B737 classic and NG. Another advantage of the dual cue is the ability of turning one off but still using the other. Specifically, I like to climb in CMD CWS mode. That's Command Control Wheel Steering mode. I get to that mode by deselecting VNAV. When I do the horizontal command bar disappears and you are left with a vertical steering bar. In this mode I control airspeed with pitch. This is a very useful mode in turbulent air, mountain wave etc. This mode results in a solid pitch attitude during climb. The -700 can be particularly pitch sensitive. One thing to note, in addition to deselecting VNAV you have to reselect N1 to assure constant max climb power otherwise the engines will maintain MCP speed.

A single cue FD does not have this capability.