Binary -verses occur when a pair of universes are formed or combined together to form a verse group or verse system. This formation can occur at higher levels of the verse chain as well, with whatever type of -verse exists at that level.
Although they are fully separate and independent of each other, they are also intimately related and, probably, physically connected in some way. When they have not been formed together in some shared origin event, their grouping is usually achieved by an event called a crossover or a merging. They can also just be grouped together by some force working at the level of the multiverse they exist in and which keeps them that way; maybe akin to a gravitational attraction that keeps them orbiting each other or a common central point or maybe similar to an electromagnetic field that keeps them attached to each other.
Like with star systems, groups larger than just two can be formed, like trinary or quaternary -verse systems or trinary and quaternary -verses.
In the same way that these groups can be formed after the creation of the participant verses, they can also break apart after some time for some reason. Maybe a large group cohesion is dependent on one of the participant verses and will dissipate if that verse dies; Maybe a collision with some other verse can tore the group apart; Maybe the stability of the group is only temporary and/or very fragile and eventually succumbs to chaos leading to the dissipation of the verses in the group.
If the group was formed by a shared creation of its participating verses or through a merging event, maybe they are physically attached to each other somehow and, in that case, they can be very hard or even impossible to separate, without causing their full or partial destruction in the process.
Travel between them becomes much easier, and beings from each -verse are often able to communicate freely.