personally, I would not propose to start 2 martial arts at the same time not having already some background. Give one at least a year's time to get used to it.
If you want to rush in grading, you might prefer one art training full time until sufficient grade (shodan?) and then add the other.
In all other cases start the second, whenever you feel comfortable with the first one. Some have problems, if arts are too similar, some if they are too different. Try what fits your needs best and have much fun