As far as I am aware of, H1 belongs to one specific job. If you change a job, you should have another H1 from new job and so on. In that situation, how can a person in X company take another job from say Y company?? Now, if you want to go illegal route, thats upto you. But, strictly speaking and having once discussed with a lawyer, he strongly recommended me not to work anywhere else unless I receive my green card. I am sharing my experience.......