Internal communication

Every body naturally hosts multiple beings, so communication between them is an important skill to master. Without solid communication, entities can cause harm to others that share their body, even without meaning to, and they risk not being able to accomplish their own goals if they are at odds with someone else's.
External communication between entities is facilitated through means such as writing notes, leaving signs or objects, or employing the help of other people from other bodies. These strategies are readily employed by all people, especially when a body family is young and internal communication is less reliable.
Internal communication, on the other hand, is a different skill entirely. It is not always so simple, and it must be practiced regularly. There are two types of internal communication: implicit and explicit.

implicit communication

Implicit communication does not use language. The full range of ways to implicitly communicate is vast, but some examples are synesthesia cues and passive influence.
For synesthesia, entities perceive feelings as other sensations such as colors or sounds, and they can use these associations to convey their own thoughts to others. Seeing purple hover in the edges of their vision could mean someone is unhappy with the ambient temperature. Whistling and whooshing sounds could reveal excitement about an upcoming party. The possibilities are endless.
Passive influence occurs when an entity changes the behavior of the body to make their thoughts and intentions known. One entity may suddenly use the body to walk toward a fountain to say that they really want a drink, actually. It may also be more subtle, such as influencing the way their hand moves when casting a die.

explicit communication

Explicit communication happens when an entity uses language directly to communicate with others in their body family. While the body itself is not speaking, entities are speaking to each other with words or sign language and engaging in conversations. In any given head, two or more voices may chatter amongst themselves without anyone else knowing.

development

Implicit communication develops first. When a body is first born and entities begin developing, they are not equipped with language. The individual senses of "I"ness rely on overlaps in their perception to communicate with each other at first.
Initial acquisition of language does not necessarily coincide with immediate development of explicit communication, however. Because early entities have experienced each other for so long, they may find implicit communication faster and more predictable.
Entities that join the body family after they become more adept at using language may be better primed for explicit communication, but because it is not necessary there may not be enough to persuade them to use it.
Children in Kugmak and Iwatik are given a lot of instruction on how to internally communicate. There are standard curricula for teaching fundamental skills that help facilitate internal communication, such as journaling and cataloging entities.

purpose

Together, body families need to coordinate to achieve mutual goals. The ability to impart information to someone else is integral to that coordination. Body families with poor internal communication can struggle to do basic things because they may not realize someone else is trying to do something differently, essentially overwriting their work at best, or sabotaging it at worst.