This idea has been merged into another idea. To comment or vote on this idea, please visit ACU-I-524 Make it possible to link multiple projects together so that they are billed on the same invoice when you run billing.
We have the ability to record Bills to different projects at the line level, but this functionality is not on the Invoice. The project is located at the header level allowing for only one invoice per project. It would be helpful to have this ability to invoice different projects at the line level just like the functionality of the Bill.