In my case, it's less monitors, and more 'two iMacs'.
Boomerang is a 2010. All the social and entertainment foo is on it. On the left.
The unnamed iMac pro is on the right, usually rendering batches of fractals. It has email on it now so I don't lose anything when Boomerang finally doesn't come back, but it requires less steady attention.
We were going to hook up the old monitor, but we're still trying to figure out how. It's stumped many a pro. If I'm being honest with myself, though, there's not really room for it unless I build it a lift to hover above the current pair, which wedge into the corner of a 70x70 L desk; I just spin the chair as needed.
For sanity, I use keyboard and mouse sharing software.