How to copy a page from one RapidWeaver project to another
Actually, it's very easy. I think the problem people have with this is that they maybe try to drag the actual page in the Edit or Preview window. That's not where you do it. You drag the page icon in the list of pages on the left hand side of the RapidWeaver window. Get both sites open and overlapping, as shown below:
Below is a screenshot of what it'll look like just before you drop it. The black line is showing you where it will be inserted; if you don't want it in that position, just drag it up and down the list until the black line is where you want it to be, then drop it.