I'm trying to find out exactly how Stances and Maneuvers work, since I plan on going as a Swordsage specializing in Diamond Mind and Desert Wind schools for their maneuvers. Is it possible to use Maneuvers from two separate schools in a single round? For example, starting in a Desert Wind Stance and activating a Boost and then switching to a Diamond Mind Stance in order to use a Diamond Mind maneuver while the Boost is still active. Would that be at all possible? Since Stances are swift actions, I'm wondering if that's possible, my DM said he isn't entirely sure since there isn't anything in Tome of Battle prohibiting it explicitly.
I'm breaking this down so we're on the same page.
"Is it possible in the same round for a martial adept to initiate maneuvers from two different schools?
Yes, it's possible as long as the martial adept doesn't exceed his normally allotted number of actions. Most martial adepts can't take more actions in a round than other creatures.
"Can a martial adept that starts his turn in a Desert Wind stance use a Desert Wind boost then switch to a Diamond Mind stance?"
Usually not. A martial adept usually takes a swift action to initiate a boost and usually takes a swift action to enter a stance, but most creatures only get 1 swift action per turn. (An example exception is the prestige class ruby knight vindicator (Tome of Battle 122-6), which can use its level 7 class feature divine impetus to take multiple swift actions.)
"Can a martial adept be in a Desert Wind stance and use a Diamond Mind maneuver?"
Yes. There is no requirement that a martial adept be in a discipline's stance to use a discipline's maneuvers. For example, a martial adept can start the turn in a Desert Wind stance, take a swift action to initiate a Desert Wind boost, and take a standard action to initiate a Diamond Mind maneuver. This is all totally legit.
I think that hits all the points. And thank you for being willing to experiment with Tome of Battle to make martial characters better.