No, you can't write both online and offline exam you have to choose one either online or offline.

It is recommended that you should always choose a mode in which your are comfortable.

You can check the advantage of JEE Main online exam below

Candidates can choose their preferred timing and date of the exam according to the slots available as per convenience.

The restrictions of OMR Sheet : You can always change a marked answer in online mode. This freedom is lost in offline mode because, well, the ink is everlasting.

Assigning extra time for bubbling : You don't have to keep a extra time to fill the bubbles in your sheet.

Disadvantage of JEE Mains Online Mode :

Technical glitches: These can occur anywhere and at most abrupt of times, especially in smaller towns with lesser technical expertise. If you feel that the facilities are not sufficient, you might want to sit this one out. Question Paper is not given to students for further reference.

Advantage of JEE Mains Offline Mode :

Reviewing the question paper : Students can go through the paper as per convenience. This helps them choose the section of their strength and begin with technical glitches or power failure fear: Since the examination is on paper, you couldn’t care less about the lack of equipped facilities or faculty.

Conventional : This mode is highly conventional owing to our education system, and hence students find it highly comfortable.Students get to take the question paper for future reference.

Disadavatage of JEE Mains Offline Mode:

Bubbling: The strenuous task of bubbling OMR, as mentioned earlier, eats up students’ crucial time which could have been used for revision. Also, it does not leave scope for correction of wrongly marked answer.Students have to appear in the exam on allotted date and time.

All the best for your exam!!