Using a secondary device while viewing a primary device (i.e. TV), or media multitasking, is now common. Numerous researchers and practitioners have attempted to introduce secondary devices into education as a new learning environment providing additional information to the user. However, the learning-related effects of using a second screen remain controversial. This study examined the effects of second-screen-application use on attention, learning performance, and user experience per content relevance for three primary contents (PC; i.e. videos) requiring different amounts of cognitive load: low, medium, and high. Second-screen use reduced learning performance and user satisfaction when the PC required high cognitive load. However, participants exhibited increased learning performance, concentration, and satisfaction with the PC requiring medium-cognitive-load when highly relevant information was presented on the secondary screen simultaneously. Based on these findings, guidelines were suggested for designing a second-screen application without degrading users' learning and experience.