I get error when adding a second email address to customer info.

Yes, you can, Asavas. In QuickBooks Online (QBO), you can add up to five email addresses to a customer's profile, as long as the total entry is under 100 characters.  

To do this, simply go to the customer's profile, click Edit, and enter the addresses in the Email field separated by a comma and a space. For example, email1test.com, email2test.com. Then, hit Save.

Since you mentioned you're seeing an error, please let me know the specific message. Also, check if it persists after double-checking the formatting. 

If you have any questions or need assistance with your additional email address, please post them here in the Community. We'd be glad to assist you further.