Eh, they can’t touch anything that’s not on their equipment, and they would have to prove that you did it while on their clock. It could be that you took your “breaks” from that job (legally allowed) to get all the work done for the other company, and you are just really productive in those 15-min breaks or whatever (or you pre-work outside of work hours and submit your work in those “break” periods). If it’s not on their hardware, they’d have a rough time proving it.