Prendi quattro uova e separa i tuorli

Hi,

In this above sentence a woman ask another to help her in the kitchen. She s ys to her " prendi quattro uova e separa i tuorli ".

My question is that i don't understand why when talking to her she uses at the same time " tu prendi" e "lei separa". So she uses tu and lei at the same time.

I would think it would be more correct to use " prendi" and " separi".

Can someone explains?