I have a similar stem on my mtb. One of the hex bolts screws into the other one.

If none of the above procedures work (but I have a feeling flanso's diagram will do the trick) try lightly tapping the top of the hex bolt with a hammer after you have loosened it to see if it pushes the other hex bolt out the other side. It may just be a bit tight.

Like dabac says, loosen it a lot first - i.e. loosen it until it's falling out then reinsert it and tap it.