First, overlap the covering. If possible, do it so that any seams face away from the airflow.

As far as how to cut it goes, a lot will depend on your design. Can you post a pic?

Finally, adding covering over covering is not so much a problem of weight, the problem is it is very difficult to get a nice finish without having a million air bubbles trapped between the two layers. I would remove as much of the old covering as possible first, or poke a bazillion holes in it so the air can escape.

Go to this page, there are two good articles on covering:

[link=http://www.rcuniverse.com/magazine/reviews/minnflyer.cfm]Mike Buzzeo Takes A Closer Look[/link]