Invoice with multiple projects

Last day of each month, I create an Invoice for every Client. On this invoice are all the projects from this client.

BUT: I can only select one project in the Invoice form, and can only see the Timesheets for this project.

HOW: Can I select multiple projects for an invoice and see attach timesheets from multiple projects to the invoice?

IF: this is not possible, do you have a timetracker where we can upvote a request like this?