Doing this could constitute fraud and/or time theft in some circumstances. If one of the employers bills your time to a customer or (worse) a government agency

Doing this could constitute fraud and/or time theft in some circumstances. If one of the employers bills your time to a customer or (worse) a government agency you might face serious legal problems. More likely the employer could sue charging breach of contract, violation of NDA, unauthorized use of company property. If your actions cause either employer to get sued or suffer damages they will likely turn around and sue you. A jury or mediator might not have much sympathy because of the deception and questionable ethics.