All martial arts have their own basic movements, distances, core responses etc. Training is largely about in ingraining those traits and habits. Trying to learn two different skill sets simultaneously confuses the brain and makes learning harder. It's almost always better to wait until you are proficient at one specialist art and then to allow your improved balance and coordination speed up the learning of a second art than to try to learn two from scratch. The obvious exception to this is an integrated training program as one might find in mma. But that is a different thing than learning judo for two hours one day then tkd for two hours another day.