Project tags are attached to invoices and currently there’s no facility to apportion x amount of that invoice to a given project, the only way is to split the invoice into two.
There are some updates planned for Projects, this ability to apportion amounts was one such feature that has come up in the past and is something we’ll look at when we have the time.
Here is the open feature request for apportioning project tags. We will update this thread in due course.