Hey,

If you still think you need a gap between the two exams the only option you have is to change your mode of exam from Pen and Paper to Online mode. You may do this using the correction window of JEE Mains.

Note: This facility is available only to students who opted for offline mode (Pen and Paper mode) and not the reverse.

JEE Mains and CBSE board exams are independent of one another in terms of being easy or hard. Since the datesheet is out and Jee Mains (paper and pen mode) and CBSE class 12 Physical Education papers are on 8th and 9th April respectively leaving no gap in between, be mentally prepared to handle it. Ensure that you start revising for both since you still have approx 2 months time. Dont keep brooding over the schedule and start preparing right away. 

All the best.