There are multiple ways to add an additional line with a consultant to the title page.
You can add a line break inside of the \author{•} macro with an additional space. As a consultant in a sense belongs to the author I would prefer this way.
\author{The author\\[0.5em]The Consultant}
If you don't need an institute on the title page, you can also use the \institute{•} macro (or any other field you don't need) and adjust the font size if desired.
\institute{The Consultant}
\setbeamerfont{institute}{size=\normalsize}
And if these simple ways still do not satisfy your needs you can always redefine your title page as described here.
\titlegraphic{One more line}might suit. – TonioElGringo Jun 22 '15 at 11:06\subtitle{Additional Line}– Benjamin Jun 22 '15 at 11:25\institute? – TonioElGringo Jun 22 '15 at 12:07\setbeamerfont{institute}{size=\normalsize}. – Benjamin Jun 22 '15 at 12:29\author{The author\\The Consultant}– Benjamin Jun 22 '15 at 12:32