You could use two binary counters, each counting the frequency of the shaft pulses from the engines. Subtracting the two counts, will give the difference in speed between the two engines. You can use a D/A converter if you want the value as a voltage.
Certainly a micro would be an easy way to do that, but it can also be done with discrete parts. You could also use the micro to control the speed of the engines if desired with the micro providing compensation to stabilize the control loop