How to navigate partners who are plural/DID:
Respect.
Communication.
Tolerance.
Forgiveness.
Equality.
Taking turns.
lather, rinse, repeat.
Much the same as other relationships, except your partner(s) or friend(s) have a zillion roommates who may answer the door when you visit. "Sorry Pat's busy in the bathroom, please have a seat. So how has work been?"
You know — you're nice to their headmates, they're nice to yours, and you kinda muck around sometimes it's awkward but you make the best of it. You play with their kids, you pet their cat, you talk to their roommates, and when your partner is finally out of the bathroom, you go out to dinner or something.
When 2 or more systems are in relationships with each other it can have some unique advantages and disadvantages.
Advantages include additional understanding of the challenges of being multiple/plural. The needs of a multiple system often are mysterious to singlets, so when multiples are in a relationship with each other it can be nice to have additional understanding. This understanding often extends to triggers, and PTSD issues. This advantage exists even when there's several multiples in a polyamorous relationship.
Another advantage is the possibility of having several individual relationships and higher variety in choice of love interests without needing to involve additional bodies. Systems with high compatibility combinations between the systems can find greater overall satisfaction or fulfillment and perhaps avoid the additional STD risks of having several physical partners.
Disadvantages include the chances of one getting triggered and their reaction triggering the other(s) in the relationship. Not all alters are kind, compassionate, understanding, calm, etc.
Another potential disadvantage is developing an unhealthy relationship with codependency or other unhealthy behiaviors — but this is a potential disadvantage that exists in all relationships, whether involving any multiples or not. There may be additional risks only because it's possible none of the multiples involved have good relationship skills or standards due to past trauma, abuse, poor role-modeling, etc.
Due to these advantages and disadvantages, these relationships can both be more difficult, more demanding of time or attention, or they can be more fulfilling. But the factors that come most into play is how healthy the systems involved are, and it's important to have high communication internally and between the system partners in the relationship structure. Communication is a big factor in all relationships, but it becomes vital here.
There's a great deal of As Inside, So Outside when you're dealing with several systems. If each individual system has high coconsciousness, good internal communication, and is developing a stronger internal community, then it improves the external relationship awareness to spot issues, the external communication between the members of the relationship, and a stronger sense of family for the participants.