Once mixed, how long is cake batter good when refrigerated?
Comments are right, mixing with another box would probably not provide enough leavening for the second cake to be successful.  Just toss the leftover batter into a smaller baking  pan.  You'd be better off having a very slightly stale mini-cake the second day than a larger fresher brick.  A bit of fruit and whipped cream would disguise the not-baked-today character very effectively.  Or you could just eat it before the guests arrive :-)