Whether you should study Quant and Verbal in parallel depends on how much time are you devoting towards your GMAT preparation. If you are studying for say 1.5-2 hours daily, five to six days in a week, it is not advisable to take up Quant and Verbal in parallel. Start with one of them and finish it, and only then move on to the next one.
But say, you can devote 4-5 hours (or even more) daily for your regular study sessions, then you can take up both Quant and Verbal in parallel, dividing the time between them depending upon your study plan. Normally we do not recommend students to take up Quant and Verbal in parallel if they are working professionals (since they can’t spare sufficient time towards their GMAT preparation in their regular study sessions).
Also, it varies from person to person. Some students are able to understand and grasp concepts better if they focus on only one thing at a time. Basically, whatever you do, do it well. So, the decision is yours.
Good luck with your GMAT preparation and let us know how we can help you achieve your target GMAT score.