Ok just to clarify I used linear polarizers in this experiment. I had the intention of stacking polarizers to create an amplified ND filter effect but ended up with an infrared shot.
I got the chance to try out some more shots earlier today. This first one is taken with a 62mm PL, 62-52 step up ring and a 52mm PL stacked on a XR Rikenon 28/3.5.
There some colour spectrum change along the diagonal which I though might have been related to the filter size difference (62vs52) as I had not noticed this the first time I accidentally discovered it with 2 52mm PLs.
I went down to the camera store and got another 62mm.
62mm PL + 62mm PL Stacked.
62mm PL + 52mm PL stacked.
So you can see there appears to be a link between diagonal colour shifts and the differences in polarizer sizes.
While this maybe an interesting alternative to an actual infrared filter, its important to note that you'll need a tripod as these infrared photos require a few seconds of exposure time. Also, you might find a lens hood handy as the extra PL filters can cause some flare in your photo!