L298N motor driver output voltage

I have a breakout board with L298N.I am use input voltage at 24Volt. I am trying to move 2 Dc motors simultaneously. I use 100% duty cycle but i do not get 24 for each channel. I get 12Volts. I seems like it divides the voltage.When i go to 50% duty cycle for one of the 2 motors i get 12 and 5Volts and if one of the 2 motors stop and the second runs on 50% duty cycle it get 18Volts.

Do you know if this is normal.Is L298N normally dividing the output voltage if there are 2 motors on the board?Or this should not happen so i have to search for the problem somewhere else.

Maybe the voltage source cannot support enough current to the motors, so the voltage drops , Please mention the Motor's Power and Voltage source's rated current.
If it was a battery you'll find it written as : (12V7AH/20HR) something like that so please post it back so i can help you.

Measure your 24V supply under full load - is it still 24V ?