Hired in two consecutive days... (I need advice)

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On Thursday, I got hired by a company that I've applied before. Then on Friday, I was scheduled with an interview by another company, and I got hired right on the spot. Now my dilemma is this: The first company that hired me is a part time or seasonal job. They already sent me paper works for me to sign and fax it back to them (in which I haven't filled it out yet). The second company, when I got hired. I sort of didn't mention or had forgot to mention that I got hired the day before with a part time job. Now what I'm worried about is the scheduling. And should I tell the company that just hired me on Friday that I also got hired with a part-time job? Please anyone, help me because I don't know what to do, and I'm about to fill up W-4s I-9s and all those important papers to be filled up. Thank you