I have to disagree, Keith. I have never heard of a 'friction tear', much less a tear caused by placing two layers of liner. Neither liner is going to move once the installation is complete. How can you have what you call a 'friction tear'?
It is advisable to cut several slits in the old liner at the deepest point of the pond to prevent water from accumulating between the 2 layers of liner. If you have rocky soil, the old liner will just give you an extra level of protection.
Sorry that you find it necessary to do this, but rest easy that you should not have any more problems with leaks after installing a large enough one-piece liner.
"Everybody is ignorant, only on different subjects." - Will Rogers