If I buy quickbooks in india, we are having two companies the first one is registered in india and the second one is in United states can I do the accounting for both co.

You should purchase 2 different versions

https://quickbooks.grsm.io/India

https://quickbooks.grsm.io/US

https://quickbooks.grsm.io/us-promo

Yes, you can, userrakeshd.

To make it easier to switch between accounts, please sign up using the same email address. This helps you see both accounts when you log in.

Here's a sample screenshot of what it looks like: 

For adding the new company, you can visit out website here: QuickBooks pricing page. Then, select the subscription you want to sign up. This opens one of two experiences:

• If you're signed in to QuickBooks, you’ll be asked to confirm the account you want to sign in with. If the account displayed is the one you want to use for the new company file, select Yes, that’s correct.
• If you haven’t signed in recently, you’ll be asked to create an Intuit account. Don't fill this out unless you want to connect your new company file to a new account. Below "Buy QuickBooks Online" select Add another company. Sign in with the user ID and password you already use for QuickBooks.

Here's a sample screenshot:

You can also read this article for more information: Create or add another company file to QuickBooks Online.

Once you're signed up, you can now switch companies.

Here's how: 

1. Go to the Gear located in the upper right-hand corner.
2. Under Profile, select Switch Company.
3. From the list, select the company you want to switch to.

Please see sample screenshot below:

Aside from switching companies, you can also open a different supported browser and log in the other account. 

I'm also adding this link with all the video tutorials to get started with QuickBooks Online: QuickBooks Video Tutorials.

If you have more questions regarding adding a company in QuickBooks Online, please let me know in your reply. I'd be glad to answer them. Have a great day!