Macbook Pro Late 2011 13" Dual Monitors
Are you able to confirm that your MiniDisplay port to 2x HDMI cable supports two separate outputs and not just mirroring of the output?

If you go to System Preferences : Displays : Arrangement can you see the two monitors?

Can you uncheck the box the says “Mirror Display”?

If you don’t see the two monitors, I’d guess that the adapter is an HDMI splitter and does not support two Independant desktops.