How to make an RC mixer with the arduino
You do not need to use long for values from pulseIn, using int will be sufficient.

Your code is not complete, at least there is a missing '}' at the end of setup.

The slower the baud rate (9600) actually the more it interferes with timing code, to a point.  PWM is all timing code.  I would suggest 57,600.

You never set the servos to any pins.  And is using Servo lib needed? I have always used analogWrite with no problems.

You are not using parenthesis on your math (currently commented). It will not give you the result you expect. But you are somewhat on the right track, subtract 1500 from each pulse read, that will give you -/+ range. How you mix that depends on how your radio is setup (servo reversing, which channel is which). Keep in mind ailerons is 1 channel, so left aileron = -rightAileron.  Elevator up = that + elevator.  so;

aileronLeft = airleronChannel + elevatorChannel
aileronRight = -airleronChannel + elevatorChannel

You do realize this a $3 part?

update: I have always used analogWrite, but I also always use good equipment. I see cautions that cheapo servos might be damaged with analogWrite. You can read more here. It has always worked for me.