In addition to two segments on the same PNR, I've always gotten credit when there are two segments on different PNRs that get me there and back.

For example, one PNR shows ATL-RDU, the other is RDU-ATL-SRQ, I have always gotten credit when the promo requires a r/t.

Since I almost always purchase o/w tickets, because my schedule changes frequently at the last minute and I usually fly from Point A to Point B to Point C to Point D then back to Point A with a middle segment requiring a different carrier, this happens pretty frequently.