Formal:
* Inseparably: This implies a close connection that cannot be broken.
* Indissolubly: This suggests a very strong and permanent bond.
* Intertwined: This highlights the complexity and interwoven nature of the connection.
* Interlocked: This emphasizes the way two things are tightly connected and dependent on each other.
* Inseparable: This suggests that the things are permanently connected and cannot be separated.
Informal:
* Tightly bound: This is a simpler way to say that two things are closely connected.
* Closely linked: This emphasizes the strength of the relationship between the two things.
* Bound together: This implies that the two things are joined together in a way that is difficult to break.
* Interdependent: This highlights the fact that the two things rely on each other.
The best synonym to use will depend on the specific context of your writing.