I used to run 2x 24" monitors in portrait mode (2400x1920) which was pretty nice.

I upgraded to 27" monitors, but portrait mode (2880x2560) involved too much head tilting so I ultimately went landscape (5120x1440), which was both too short vertically and too wide horizontally.

So I'm currently using a single 30" (2560 x 1600) in landscape mode. It's ok. There's simplicity in having a single monitor.

The next (and possibly final) upgrade will likely be to a ~32" 4K or 5K monitor. If I had a 2nd monitor, I'd probably just want it to mirror the first one at 3x zoom centered on the cursor - my eyes are losing resolution about as fast as monitors are gaining it.