My experience has been that booking a single itinerary with multiple airlines doesn't always trigger the credit when using an Online Travel Agency (OTA) such as Orbitz. The reason for this is that in order to trigger the 5x credit, the purchase must appear on your card statement as if it came directly from the airline. For itineraries which involve a single airline, using an OTA generally works (see caveat below!) because the OTA tends to pass the payment information along directly to the airline for processing. When booking an itinerary with multiple airlines, OTAs may instead opt to collect the payment themselves, later distributing the collected payment to the airlines during a separate invoicing process. In this scenario your card statement will show a charge from the OTA, not from the airline, and the transaction will be ineligible for 5x points.
An important caveat to my earlier statement: even though I said itineraries on a single airline generally work due to how OTAs tend to forward payment information, this is by no means guaranteed. The OTA could at its discretion decide to collect payment itself even for these itineraries, which would cause the transaction to be ineligible for 5x points.
If you can get the fare codes of the itinerary you saw through Orbitz, try giving Amex Platinum Travel a call and seeing if they can price out the itinerary similarly. I've had success with this when trying to get specific fares that don't show up on Amex's own travel web site. There's a small booking fee for airline tickets done via phone, but you're guaranteed 5x points since you're going directly through Amex Platinum Travel.