Can we open two mail id in one browser?

you can sign-in different email accounts in one browser. you just have to click the "Add Account" button on the google account page to sign in to your other accounts and conveniently switch between them.

when you sign-in different email accounts in one browser by above said method, there will not be any merge of data and access to one another accounts.

you can learn more about this here,
https://support.google.com/accounts/answer/1721977?hl=en