On many routes, two one-way's will cost more than a roundtrip award booking. Domestically, it will generally depend on whether or not there are revenue fares on route with roundtrip booking requirements or not (frequently found out of hostage hubs which don't have much LCC competition like DTW and MSP). It's almost always the case on cheaper international awards. Further, if it is international, one-ways and roundtrips to the US will often get hit with international surcharges. You can tell if you have awards with roundtrip booking requirements as Fare Details: line in the email receipt will list the award fare basis codes. Award fare basis codes with an 'R' in the middle of them are ones with a roundtrip booking requirement. As noted above, they will reprice the one-way at whatever one-way award is available (which will be based on award fare rule requirements and award level bucket availability of the various one-way award levels).