That depends on what kind of plane your doing. On a slow flying plane, one side is more than adequate. Something fast, like a wing, and it may be better to do both sides.

There are a few other ways of hinging I have used on foam also. Small hinge points work great, as due CA hinges on some kinds of foam (foam safe CA of course). Even GOOP or regular old low temp hot glue can make a really strong hinge. Just put it on and smoooooth it out over the hinge line. Some people even take the "disk" out of floppy disks, cut it up and use it as hinges.

Laine.