Check out 6502x86's PTime thread. It is actually a pretty good theory regarding the DBT in that the DBT can contain information and spontaneously not exist in the universe at the same time.
Lavos/Schala are exceptions, and likely because both of them combined have enormous power over time and space.
This is something always bothered me. The DBT holds dead timelines, right? These dead timelines contain static information. Essentially changing them wouldn't make any sense. There isn't a DBT for the DBT right?
When Schala merges with the Lavos from the static, dead Lavos Timeline - she changes him, they can't grow in a dead timeline together, so I would think that they simply "exit" the Lavos Timeline.
While the Dream Devourer is obviously inside the DBT, it isn't inside a dead timeline, it exists in the space outside, where the dead timelines reside. This is my explanation for having to enter two gates to access the DD in CTDS:
1. The first gate takes you to 1999AD in the dead timeline. Here, Eclipse Magus tells us that this is the future where Lavos has been defeated. yay support!
2. The second gate takes you to the space
outside of the dead timeline. This is where the DD resides - what was once static information has been changed, squeezing the new information out of its home timeline.
Of course, Serge accesses the "outer space" directly in CC.