If you have a function generator that has independent control of frequency and pulse width, connect it the Clock input of a positive-edge triggered D Flip Flop. Wire the Qbar output of the DFlop to its own D input. Q and Qbar are complementary.
Now if you want to shorten the two complementary outputs to be the duration as the pulse width of the function generator, wire two AND or NAND gates so that the Q and Qbar outputs are gated by the function generator signal. Now adjusting the pulse width on the function generator gives you control of how long the two outputs last.