How to connect 2 external monitors to MacBook Pro 2015
With an MacBook Pro Early 2015 you can connect two display using Thunderbolt 2


  2x up to 3840x2160 on external displays via Thunderbolt 2


or one display using Thunderbolt 2 and a second one using HDMI


  1x single display up to 3840x2160 via Thunderbolt 2 
  
  1x single 1080p
  display at up to 60 Hz, 3840x2160 at 30 Hz, or 4096x2160 at 24 Hz via
  HDMI


So in your case you need a Mini DisplayPort to HDMI adapter for a second HDMI display. If you want to use display with a higher resolution you need to get another adapter (DVI, DisplayPort, etc.).