: Is that true or false, or does layering in and of itself add strength that a
: single layer doesn't?

From a practical standpoint no. From a commercial standpoint, it's easer to lay one layer than two.
From an enginerring standpoint, my guess would be that the ultimate strenght would be roughly the same, but the layered approch would be stiffer. (that is assuming that the glass is less elastic than the epoxy)

: it's just that the tape is easier to handle, particulary on the
: interior when feathering your edges.

cutting you own is cheaper than buying bi-axial tape (esp. when you are using scraps) My experiance is that the tape has something in it to keep it from fraying, which while it works, means more effort spent in fairing it away.
One tape I bought at Fred Myers has what appears to be a nylon thread in it to keep it from fraying, this is visibal through the epoxy (if you know where to look)