You can work for the next employer, but if the original employer revokes the I-140 within the 180 days you're out of luck. And if the USCIS sends an RFE within the 180 days, questioning your employer's intent or ability to hire and pay you, you may have to show that your original employer still intends to hire you once you get the green card. Basically, you have to ensure that your original employer will cooperate for the 180 days. Which is very risky once you have left them.