You wouldn't "fire" up a bundle, rather you would start a playback fader or button and should you transition into a different playback on the same bundle number, it would automatically "release" the first playback. In another words, when you bundle together playbacks, it makes transitioning between them easy and will never let you have two playbacks on the same bundle run at the same time.
As far as stack-lists are concerned, after assembling a stacklist with multiple cuelists attatched to it, you have to assign it a playback button or fader. When you trigger the fader or playback, more than one cuelists will run at the same time. If you use a stacklist on a fader, it will give you a dedicated dimmer for that stack.