Unless you script things, the audio elements run at the same relative volume.
I do all my mixing in Audition with a project containing all audio content on separate tracks. Depending on the cue, I use the various background music as a bed and set the levels for other cues relative to the bed. This also makes it easy for matching levels of various background music selections side-by-side.
In addition to using this to set relative levels, I will also adjust the EQ on the cues so that they can cut through the mix without having to kick up the volume.
When I'm done, I'll solo the channels and output them individually.
I don't output in stereo unless I have a good reason, such as something that simply must be panned.