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If you want to cast off 2sts, go ahead and knit one more stitch and lift a stitch over and off the needle. That'll give you one stitch on the right needle and 9sts on the left.
(Since you've decreased the number to start with from 15 to 12, casting off just the one stitch may be a good idea. That would mean one stitch cast off, one stitch on the right needle and 10sts on the left.)