I have a shawl I knitted many years done similarly on two different sized knitting needles. The stitch was a bit more complicated than straight K1 P1. I seem to remember it was called blackberry stitch, and because of the uneven and large size of the needles, it came out loopy and lacy. I still have the pattern for it.
This sounds like a great idea. I love it when people "freestyle" with knitting. Re the casting off, I would do the final row before casting off with both needles if the same gauge (the thinner one), thereby avoiding cast off problems.
A fellow member of my knitting group was using an old pattern and making a V neck sweater for herself. She was using a size 10 needle and another very much thicker needle. The pattern was simply K1 P1 and the needles used alternately were giving it the texture. You learn something new every day as I've never seen this done before. Has anyone ever done this, and could a normal pattern be adapted to this stitch?