Ideally, we do not recommend a working professional to study Quant and Verbal in parallel as it is difficult for a working professional to devote a significant amount of time for his/her preparation.

Basically, 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.

However, if 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. 

The bottom line is that you need to make sure the quality of your preparation in either section is not hampered by your decision to prepare in parallel.