2014 Mac Mini with two mDP-DP monitors
You will not be able to drive a Mac Mini to a monitor with a resolution of higher than  1920 x 1200 off of that HDMI port (different versions of HDMI have different capabilities vs. different versions of DP/mDP--the Mac version of HDMI is old). For 2560 resolution you must attempt to use the Thunderbolt ports to connect to the display mDP ports--in theory, although they are not identical, as the thunderbolt capabilities of data transfer are a superset of the mDP's, the thunderbolt port is backwards compatible with the mDP port and the Mac Mini will recognize the mDP monitors. Problems have been reported in practice in achieving full resolution for the mDP displays; there are also positive reports.

see: http://www.techadvisor.co.uk/feature/digital-home/displayport-vs-hdmi-3535257/
and:
https://support.apple.com/en-us/HT204388