Ok. PJSim is correct. You change households in Edit Town, off the main menu. But, like I said, you will lose all the promised wishes of your first Sim, plus everything in the refrigerator. That was a big deal to me because I was storing all my perfect fish and produce in the refrigerator for my 10th generation Legacy family and lost ALL of it when I changed households! Also, your first Sim will continue with his/her life while you are playing the other Sim. If you have story progression and aging turned on, your Sim will be different when you go back to him/her. Not different traits, but experiences that you weren't there to see. You might come back and find that he's gotten married! Or something like that.
I hope this helps!