Tuxedo!

After altering the shape of the shirt so it looked like there was a jacket over it (adding a lip wherever the jacket overlaps the shirt), I needed to work out how to add multiple materials to one object. After a while of searching, the solution was simple!

In the material editor, click 'standard' then select 'multi/sub-object'. Click set number to choose how many materials you want the object to have and edit them in the normal way below. To assign them, select a face on the object and scroll down until the section where material numbers can be selected. Change the numbers depending on which material you want that face to have.

All I need now is the animation rig and a head. Until then, this will have to do...