A sleep regression is a stretch of time when a baby who had been sleeping reasonably well suddenly starts waking more often, taking short naps or fighting bedtime. It usually lasts a couple of weeks and then eases.
Despite the name, most regressions are driven by development. A leap in skills, a growth spurt, teething or a shift in how much sleep a baby needs reorganises their sleep for a while. The famous 4 month one is not really a regression at all: it is the point where sleep matures into more adult-like cycles, which is permanent progress.
With twins, regressions often land on both babies around the same time because they are the same age. Keeping the day consistent and avoiding overtiredness is what carries you through.