I can't really help you, except to describe what the 2 cables do...

One is a straight throttle cable to the carb or throttle body. The second is the shift cable to the tranny. The throttle lever at the driver's seat controls how the cables interact...i.e if you push the center button it holds the tranny cable in neutral, but allows the throttle cable to move normally. Otherwise, the lever actuates the tranny shift before allowing the throttle cable to move. Some (Merc's for one) have an instantaneous ignition cutout switch during shifts to prevent loads on the tranny. My Supra does not have a cutout.

It sounds like maybe one of your 2 cables got hot and is binding.