Mixing two fluids dynamically using valves and sensors
Expanding to an answer, 

Here would be the example circuit from falstads circuit simulator, its not a perfect sum, but it would fill the role, the value of the potentiometer controls how wide a variation you can control, you would just use a push button to trigger both after selecting the ratio you want.



Falstad Circuit Simultor Link

Failure point would be if the potentiometer went open circuit the circuit would never turn off, for this you could use a third fixed monostable to supply a ground to your solenoids so even if it gets stuck on, it acts as a timeout,