I could not find 2ft x 2ft plastic lens es for my lights so I bought 4ft x 2ft and will cut them in half except they started cracking.

You need an acrylic cutting knife. It is a knife with a very small hook end that grooves the sheet so it can be snapped. Definite PIA to cut sheet, particularly cheaper styrene.

Possibly with the material clamped between two pieces of plywood or other wood, then using a paneling (fine) blade placed backwards (front of blade becomes the back side) in a circular saw..
That is how I would try to cut it, like the way I have cut some types of plexiglass but I have never tried it with the lenses you have described. Wear safety glasses in all cases and good luck.

I have cut that cheap material and you need to clamp it and use a plexiglass knife to cut it. Make sure and score the material almost all the way through before trying to snap it in two. Any error and it will break wrong.