How to combine two values in one pivot table

Hi

I have a list of projects, with an invoice date, and an invoice value. I pivot these with the project name up the side, and the dates as months along the top, with the invoice values in the Value field. Much like a calendar view.

Some projects have two invoice dates (start and finish invoices) with two invoice values. These have two invoice values. 

I want to have both invoice values shown on the same pivot table. That is in May have £200 shown and in Aug have £500 shown.

Is that possible? The best I can do is have two pivot tables, one for each set of invoice data, and then I have another table which adds up every cell, but it's a pain. 

Olly