Ancient cities can generate so close together that they're touching / indistinguishable, giving the appearance of a massive ancient city with two centers.

Version: 22w16b
Seed: -5577804960544527884
City Center 1 Location: /tp @s 1580 -31 -95
City Center 2 Location: /tp @s 1377 -21 -932

Left and right sides of following picture are separate cities

Left and right sides of following picture are separate cities

(bugged) city center in foreground, with bottom of second city center visible in background
(city center submerged in lava aquifer, tracked in MC-249784)