We are setting up a new line of business. And we want to track it's profitably separately from our existing business. What is the best way to set up QB?

You need to signup for a new QBO Plus account for your second business or consider switching to QB Desktop to manage multi company files. 

Hi there, yuliana.

I'm here to help set up your QuickBooks Online (QBO) for your new business.

Fiat Lux - ASIA is correct. In addition, since each company file has its own paid subscription, you can add another company under the same QBO account. You can access them with the same sign-in info.

To add a new company:

1. Visit the QuickBooks pricing page and select the subscription option you want.
2. Enter the email address you use with your existing QBO account.
3. You’ll be prompt that the user ID is already taken. Click the Sign in link and sign to your QBO account. 
4. Fill in the needed information about your new company and you’re all set.

If you have multiple companies under separate accounts in QuickBooks Online, here's how to move them all into one account. Refer to this article for more information: Sign in to multiple QuickBooks Online companies with one user ID.

Whenever you sign in, QuickBooks asks which company file you want to open. To switch between companies, select Settings ⚙, then Switch company.

Moreover, if you don't want to sign up for another account, you can utilize the Class/Location feature (for Plus and Advanced versions) to manage or track your businesses separately. To know more about this, you can check out these articles for future reference:

• Turn on class tracking in QuickBooks Online.
• Set up and use location tracking.

Please leave a comment below if you have follow-up questions about this or anything else. I'll be more than willing to assist. Stay safe.