You have probably already completed the project by now, but my suggestion would be to use the whisker switches as a trigger input(s) on the 556(dual 555) as a (Couple of) 1 shot(s)(aka monostable mode). The output of these could be used to control (power) the 555(s) which could be set as oscilators(aka astable mode) the frequency they produce could be mixed by diodes (arrow pointing away from 555 pin 3, twards the servos) the signals would both be able to get to the servos depending on which 555 had power. Info on astable/monostable configuration and the component values needed for the frequencys can be found here http://www.see.ed.ac.uk/~kap/Hard/555/node2.htmlhttp://www.see.ed.ac.uk/~kap/Hard/555/node2.html

Good luck and have fun!