Make motors run the same speed using encoders

[IisMathwizard]

Im sorry but i don't think i understand your question well enough. From what i got it sounds like your having problems with making the robot go straight even if the joystick is tilted in an off direction. If this is your problem then you can make a case structure hook up to the button your going to use to initiate this coding (im assuming a button) and in the true wire the x axis input on arcade drive (yet again more assumtions) to a 0 constant in the case structure and for false have the x axis output go through the case structure to the x axis input. It MUST go through the case structure.
in boolean algebra it would look something like this:
a = button;
Xinput = axis input;
Xoutput = plugged into the arcade drive;