One of the major themes in this book is a specific berry/fruit that, when eaten, causes the person who eats it to slip into a coma and have an out of body experience. They can then float around at will in their incorporeal form. However, there are limits, whoever eats the berry/fruit has to "re-enter" their body within a certain time period, or their body will "die" (because of the coma state of their physical form). The more berries/fruits a person eats at once, the stronger the effect will be and the faster/further they will be able to travel in their incorporeal form, however the more dangerous it is (as the berries/fruits are highly toxic).
There is also a scene in the novel where a the incorporeal form of a female (one of the main characters, taking the coma-causing berries/fruits to use as a tactical reconnaissance maneuver during a war involving dragons) meets up with the incorporeal form of a male (also a main character and love interest(?) of the female) who is also taking the berries/fruits (I believe to find the female as she has not returned to her physical body in some time). From what I remember, they are both above a battlefield in this scene, and they cannot find their way back to their host bodies. So, they "re-enter" themselves into the mind of a dragon instead of their host bodies, and promptly proceed to essentially have sex inside of the dragon's mind.
There are several other scenes/plot lines leading up to this part of the book, from what I remember the specific scene above was near the ending of the book.
I know this book was quite thick, and may have been a compilation of multiple related stories or possibly a series compilation.
I've already taken a look at the Pit Dragon Trilogy, and the Pern novels, and those aren't it.