In BQE CORE, you can bill different phases of a parent project or different projects of the same client on a joint invoice, also called a combined or consolidated invoice. You can decide to always generate a joint invoice by setting that rule Send as Joint Invoice in the Clients or Projects screen, or do it spontaneously as part of the billing process in the Invoices screen.
To combine multiple invoices on a single invoice, use the Invoices screen from the side menu > Billing. Check CORE Help Center for details on joint invoices.