The Samsung EVO should be classed as your primary drive because it has the OS installed on it (presumably). That drive should have your OS, applications, and a couple of frequently used games.
The Seagate will act as a storage drive for media files etc, and for any games and applications that don't fit on the Samsung. By default everything will go to the Samsung drive, but when you download or save files you can choose to send them to the Seagate drive.