I'm filling my taxes for 2 different jobs (Worked on both at the same time throughout the whole year). My tax refund is being smaller with 2 jobs than just and I file ONE

That is normal and is the way it works.

WHY DID MY REFUND GO DOWN WHEN I ADDED ANOTHER W-2?

You started off with your first W-2 and your refund looked high?  Then you added another W-2 and it stopped looking so good? That is normal.  When you added more income, your tax liability increased, so you saw your refund decrease.  The program began by giving you your standard deduction—- which lowered your taxable income. So you are not being taxed on as much of the income on that first W-2.  Then you added taxable income--so the refund went down. Your refund (or tax due) is based on the total of your income, not “per W-2.”  Wait until you have entered ALL of your income and credit/deduction information.  You can't really tell anything until it is all entered.  That “refund monitor” does not mean anything until everything has been entered.

https://ttlc.intuit.com/questions/2273878-why-did-my-refund-drop-when-i-entered-another-w-28