Assume for simplicity that we mix half coffee and half creamer. This halves delta t. However the area will not double, since the top and bottom areas are the same. Furthermore, as another commenter pointed out, the top area is where most of the action happen.

However, if we really want to overcomplicate things we could consider the possibility of an insulating air pocket in a half-empty cup, leading to less convective losses. Consider a vacuum flask half full of hot coffee outside in a strong wind. If you fill it to the brim with creamer it might cool faster. Evaporation might become important too.