I think we can use PinChangeInt
One Digital input for each FlowSensor
Lets say the variable to hold Fuel Consumption is 'fc'.
First PinChangeInt (The forward) will add 1 to fc, and the second (The return) will substract 1 from fc
The main loop only check for time periode.
When the periode reached, calculate the real fuel consumption basd on fc and your sensor spec (pulse/galon). That if you want consumption per time
Alternatively, if you easy about acuration, you can put freq devider (prescaler) between the Forward-Sensor and First PinChangeInt.
I'm pretty sure soon you will also want to look at the RPM.
Have a nice hack
-bino-