I'd be happy to provide information about invoicing systems in QuickBooks Online.
QuickBooks syncs with thousands of popular business apps.
You can link two invoicing systems in QuickBooks Online, just make sure that it will not produce or create the same invoices to avoid duplicates.
You may also wish to contact the third party app invoice system provider, to verify on how does it works when integrated in QuickBooks Online. This is to ensure that your transactions will be accounted properly.