Both the companies deny sharing offer letter over email. They are asking me to be at their office to get offer letter.
It's a commonly followed practise among mid-sized IT companies in India. It's apparent that neither of the companies want you to shop around using their offer letter for a better pay. In such cases, the candidate is typically handed the offer letter once they are present for completing the joining formalities.
Generally speaking, if they have denied sharing the offer letter email, there is no way you'll able to convince them otherwise (as should be apparent from the aforementioned thought process).
If you have a preference for one of the companies over the other, and have made up your mind to join them, seek a joining date and be available for completing the joining formalities.
Make sure to go through the job terms and conditions and the mentioned pay very carefully in your offer letter. Sign the letter only after you are convinced with the applicable terms and conditions.