1st check your brake fluid level in your master cylinder. Low fluid level in the brake fluid reservoir will cause both lights to come on. If it was really low you should fill it up and then bleed the brake system. If fluid level is ok, then a failure of a component in your anti lock brake system has occured.
Check the brake light fuse. If you have 12 volts at the brake light switch I would suspect the signal light switch is at fault.
brake light switch would be most common cause
The most likely cause is low-brake fluid.
More than likely it is the brake light switch that is attached to the brake pedal.
Is it the parking brake light or the ABS light? is it red light or amber?
It is possible that a wheel speed sensor is bad. This will cause the anti lock braking to not read the wheel correctly which would be a brake problem that would cause a light.
A short in the brake light circuit.
i have a 86 bronco that cuts out when it reaches running tempature.
Low brake fluid, pads worn down, other problems.
short circuits in the turn signal and brake light circuits
On your brake peddle there is a switch it is either bad or stuck, most likely.
check your brake fluid levels