Regardless of the actual attributes of the tribes, there are going to be several potential starting points from which the involved tribes occupy the same area.
A single tribe has many off-spring. Enough so that there is an entire group of people who are not necessary for the working of that tribe, and have enough people to start their own. They could split under either good or bad terms.
- The tribes who do this will have a number of similarities to start with
- They will also have working relationships (either good or bad), unless one migrates out of the area for a long period of time.
- As they continue to grow and split, you could end up with many similar, but not the same, tribes in family groups - each may have feuds or alliances with other families.
A tribe migrates into the region. This could be simply because the migrating tribe was following their food source, or it could be because of a natural disaster or other shortage in supplies, or maybe the grass is just greener here.
- A migrating tribe is under a lot of pressure to find resources and provide for the tribe. Depending on their views, and the amount of hostility between them and the local tribes, will determine how the relationships turn out. (I don't have any facts on this, but I would imagine a migrating tribe entering the area is usually frowned on)
- A migrating tribe is likely to have very different attributes from the ones in the area it is entering. The larger the physical distance traveled, the more likely that they are different.
With those two types of tribes, you could imagine how a group of tribes might occupy an area and suddenly another, very foreign, tribe shows up. The foreign tribe migrated due to a disastrous storm and hurricane which pushed them out of their normal habitat, which is essentially destroyed.
Seeing people in distress, the tribe who first came across them may help, and the migrating tribe might pick up some survival tactics from the natives for this new habitat, completing a friendship. However, the migrating tribe settles down nearby in a different local tribe's area. This other tribe doesn't take kindly to the strangers and is offended by their worship/fear of storm gods, so try to push them out.
You can see how eventually, the migrated tribe might have officially moved in and taken over one of the other tribe's spots, yet have good relations with that first tribe who is totally different from them.
You could also see how the first tribe might align with their neighbors, and the migrating tribe might be forced onto lands that can't support them - so are required to raid for their food at times.
It's all going to come out of how the tribes split up when growing, or how they first migrated into the region. The farther back you flesh out a tribe's history, the more you'll come up with various beliefs and traits for that tribe, making it less and less flat the farther you go. It is the history and the "why" a group is the way it is, that makes it less flat. Even if, story-wise, you tell this difference via their myths or beliefs rather than explicitly stating the history.