Hello aspirant

I hope you must be doing well.

As you asked, You can prepare for both jee mains and advanced at the same time as syllabus is somehow same for both.

Go through the topics in details and give mock tests, test series, materials given by institutes. Questions in advanced becomes difficult as compared to mains.

Prepare well for mains. You can see that some of the parts of advanced is from mains also.

I hope It helps you.

Feel free to ask your doubts if any.

Good luck