Each you search "wavefront" in this forum you might find some VERY useful threads about the topic.
As far as I know they switch between clauses, if needed and also they run in parallel between instructions. For example, it seems there are two slots (odd and even) for wavefronts on each TP, my limited assumption is that two wavefronts map to a SIMD at one time (more WFs get mapped to that SIMD, if registers allow it's just that they don't run "at the same time" but they do "run in parallel" and by "parallel" they mean "switched between clauses"), the same quad from each wf gets mapped to a TP on that SIMD and the threads in those quads get executed in parallel, odd and even.... this is how it was explained to me (I think, lol), but like I said, please search "wavefront" in this forum and you will find more.