How to record time on a multiple projects / customer at a same time?

You can create a weekly timesheet to add multiple projects/customers. I'll provide the steps below.

To enter the total number of hours worked by your employee or supplier for the week, refer to the following instructions:

1. Go to the + New and select Weekly Timesheet under Employees.
2. Using the small arrow ▼ icons, choose the name of the employee or supplier and the week you want to record.
3. Determine who you want to bill the activity to or track expenses on the Choose a customer or project box.
4. Complete the rest of the fields.
5. Click Save or Save and close.

Additionally, you'll want to adjust the available days on the Weekly Timesheet by selecting the Settings below the Total Hours. Check out this article for details: Enter a weekly timesheet in QuickBooks Online.

Furthermore, ensure you turn on billable expenses to create invoices from the timesheets if you plan to send a sales form to your customer for project hours.

Recording the time of various projects/customers at once is convenient. I'm just a few clicks away if you need anything else about hourly labor and profitability.