Dell E2720HS flickering as external MacBook Pro monitor

from monitor menu (the buttons) change the MST setting to Primary and Secondary accordingly

Mactracker lists a 2012 MBP as a 13 inch Retina model (MD212LL/A, MacBookPro10,2) with an Intel GPU and using up to 1 GB of shared memory.

Mactracker also says:

Supports an HDMI-compatible device while using one Thunderbolt display or support for two Thunderbolt displays

If the model you put down is indeed this one, then you'll need 2 Thunderbolt displays. Or you can run one monitor through the HDMI connection and another one through the Thunderbolt connection.

What this model can't do is run two HDMI monitors which it sounds like you're trying to do.

I highly recommend both everymac.com and Mactracker as sources of info on older Macs. Both have a wealth of information which would have given you the answer(s) you've been looking for.