How to import this? Customers have both Street address 1 line and Street address 2 line, yet sample spreadsheet for importing customers only includes one address line.

Hello there, lorif.

I can share insights about importing customer's street line address in QuickBooks Online (QBO).
 

When importing customer's address in QBO, the mapping column for your customer's address will only include one Street line address. There may be a Street address line 1 and Street address line 2 in the customer's profile but they are the same. If you click the Add lines dropdown in the Billing address section the Street address 1 and 2 will automatically combined.

In case, you want to import customers in QBO and they have Street address line 1 and Street address line 2, then you'll need to combine them in one street column and separate them with a comma.
 

To import customer, here's how:
 

1. Go to the Gear icon. select Import Data.
2. Select Customers and Browse to upload the file from your computer. Choose the file, and select Open.
3. Or, to upload from Google Sheets, select Connect to sign into your Google account. Choose the file, and then Select.
4. Hit Next.
5. In the Your Field column, select each small arrow ▼ icon. Then, match your field to the corresponding QuickBooks Online field.
6. Select No Match if you have a field that doesn't have a matching column in your spreadsheet.
7. Confirm your contacts. Uncheck any that you don't want to save.
8. If everything looks good, click Import.
    

I'm attaching this article for future reference in case, you want to import your customer or vendor contact lists from Outlook, Excel, Gmail, or Google Sheets: Import customers or vendors from email contacts to QuickBooks Online.

Feel free to post a reply if you have clarification with importing customers in QuickBooks. I'm always here to assist you.