1. You can file two H-1B petitions simultaneously. When you are filling a quota H-1 more than one company can file, but the company shouldn't be related to each other. If the company is related you have the risk of being denied both applications. But if they are totally different companies you can have as many as three thousand H-1's filed and nobody cares.
2. Look at form I-129 and the supplement H. For sure the first company who has filed would not know and I don't think so any of the companies would find out about the other company. It depends entirely on the language of the questions on form I-129 and supplement H. You can double check this. They don't require to disclose pending filings only approved cases.
3. If both H-1B's are approved, you can join any company. One company's approval does not overrule the others. The only thing I would say is to be careful if you have signed some kind of a reimbursement agreement that you will reimburse them X dollars for liquidated damages if you don't join. You might have a legal issue from the contract side, but from the immigration side, I don't see a problem.
4. I don't think so unless they want to check if the companies are related that could certainly be a possibility. That's one way they could find out. More...
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