Here is a link to a few twin trunk bonsai in the gallery. Some better than others. A successful twin trunk bonsai may give the impression of an adult and child with the more mature tree providing "protection " to the younger trunk.
http://www.memobug.com/cgi-bin/imag...&boo l=and
Here's some more
twin trunk trees from the newer museum area
In addition to the very good advice Joao_Felipe provided,
twin trunk bonsai can also be developed from two separate trees of the same species. _Ideally, one would be a cutting so that its leaf and branch character would be identical to the "parent" tree.