Once mixed, how long is cake batter good when refrigerated? After using a cake mix to bake in a lamb shaped form, I have batter left over. How long can I keep it refrigerated (not frozen) and still combine it with another cake mix batter -- and have the second cake bake properly?
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 :-)