Nope, you will have to select between one or the other. Which ATI card are you trying to use? There shouldnt be a reason to need the intel graphics if you have a discrete card, the card will support two monitors and be significantly faster.
The 5450 isnt too old, and its significantly better than most of the intel integrated graphics and about even with the best they have, the HD 3000 on the 2500k and the 2600k, so even if you could make the intel IGP work with it it wouldnt help out much.