Just think about it. 5-perf 65mm is one-perf taller than 4-perf 35mm anamorphic but twice as wide more or less.
IF you want a 2.40 : 1 image and your anamorphic lenses have a 2X horizontal squeeze, then you are using a 1.20 : 1 area of the negative to achieve your 2.40 : 1 image. Keep reminding yourself of this fact every time you think about 2X anamorphic photography.
This is the shape of a 1.20 : 1 area, whether or not it is on film or on a digital sensor, it's an aspect ratio:
If the 5-perf 65mm negative is 52.63 x 23.01mm then a 1.20 : 1 area is 27.61mm x 23.01mm -- that's almost cropping the negative in half horizontally.
To put it another way, the 5-perf 65mm negative is already nearly 2.40 : 1 at 2.20 : 1 with normal lenses so you don't need anamorphic lenses to achieve a 2.40 : 1 image.
And another way to look at it is that 4-perf 35mm is 17.5mm tall so you are basically only gaining the increase that a 23.01mm tall 65mm negative will give you (i.e. one more perf vertically). So that's a 1.3X larger negative but you had to crop your 65mm negative almost in half to get a 2.40 : 1 frame, so you managed to pay twice as much for film more or less and then throw almost half of it away again.
Now if you wanted to achieve an even wider aspect ratio, then anamorphic lenses might make sense. With a 2X anamorphic lens, 65mm becomes a 4.40 : 1 aspect ratio, pretty wide!
This is why the 65mm Ultra Panavision format used a mild 1.25X squeeze to fit a 2.7 : 1 image onto the negative. This was originally for making 3-panel Cinerama prints, which is a 2.66 : 1 format. "Star Wars: Rogue One" used those old 1.25X lenses on an Alexa 65 digital camera but cropped the image back to a 2.40 : 1 frame. See:
There was a large film format that used 2X CinemaScope lenses designed to cover the larger film area -- it was called CinemaScope 55 but in this case, the negative was nearly square-shaped because it used anamorphic lenses. See:
Most 2X anamorphic lenses won't cover a larger negative since they were designed for 4-perf 35mm.