Can a Macbook Pro A1398 drive two Dell U2515H and at what resolution?
According to Everymac…


  This model supports a simultaneous maximum resolution up to 3840x2160 on two external displays via Thunderbolt 2. Alternately, it can support a single display up to 3840x2160 via Thunderbolt 2 and a single 1080p display at up to 60 Hz, 3840x2160 at 30 Hz, or 4096x2160 at 24 Hz via HDMI.


According to Dell..


  For convenient connections to other devices in your workspace, the U2515H provides two HDMI (MHL) ports, a DisplayPort, a mini-DisplayPort and five USB 3.0 ports. 


So, theoretically, both over Thunderbolt > Mini DP
or one over Thunderbolt &amp; one over HDMI at 30Hz