Dual external monitors on Dell E-Port II docking station are mirroring with each other

I have a Dell Latitude E7450 running Linux Mint running Mate and I'm using a Dell E-Port II (Model PRO3X) docking station. I'm connecting two external monitors, one via DVI and the other using Display Port, for a total of three displays. I can't get the external monitors to be independent and they are mirroring each other. I have no issues with the laptop's display itself. I've tried the proprietary driver and that didn't seem to help. In Windows 7 there is no problem. This is the output of xrandr, which shows the dvi and displayport as "DP1", regardless if one or both are plugged in. I can't get all three to show up when using the docking station. Screen 0: minimum 320 x 200, current 3520 x 1080, maximum 32767 x 32767 eDP1 connected primary 1600x900+0+180 (normal left inverted right x axis y axis) 309mm x 174mm 1600x900 60.0*+ 40.0 1440x900 59.9 1360x768 59.8 60.0 1152x864 60.0 1024x768 60.0 800x600 60.3 56.2 640x480 59.9 DP1 connected 1920x1080+1600+0 (normal left inverted right x axis y axis) 477mm x 268mm 1920x1080 60.0*+ 1280x1024 75.0 60.0 1152x864 75.0 1024x768 75.1 60.0 800x600 75.0 60.3 640x480 75.0 60.0 720x400 70.1 HDMI1 disconnected (normal left inverted right x axis y axis) DP2 disconnected (normal left inverted right x axis y axis) HDMI2 disconnected (normal left inverted right x axis y axis) VIRTUAL1 disconnected (normal left inverted right x axis y axis)