Yes.You can use two whatsapp accounts in your single phone. That's very easy .Follow bellow steps.
STEPS:-
1) Uninstall your whatsapp application.
2) Now just download this New whatsapp called OGwhatsapp.
* And press 'click here' link.
3) After downloading just install it.
4) Now just simply accept their terms & conditions.
5) Now Enter your old mobile number that you had used for your whatsapp.
6) And simply verify it .Thats...