There are two different DVI connections on your graphics card.
Look at this chart here Video connections. Your main DVI connection will be the second one down and your secondary DVI connection will be the fourth one down. Note the four pin holes missing between the two. I suspect you have bought two DVI adapters ,where as you should have bought one DVI-I adapter and one DVI-D adapter. These can be picked on fleabay for a little over £2.