Halliday & Matthiessen (2014: 575):
A clause containing a verbal group complex is still a single clause, and represents a single process. It has only one transitivity and voice structure.
If it is a paratactic complex, this process consists of two happenings – two actions, events or whatever. If the verbal group complex is hypotactic, on the other hand, there is only one happening. Thus in a paratactic complex each verbal group has a definite voice, although the voice must be the same in each case; but in a hypotactic complex only the group that expresses the happening, the secondary group, actually embodies a feature of voice. The primary group is active in form, but there is no choice involved. (The exception to this is when the clause is causative.)
The different types of hypotactic complex have different potentialities as regards the passive. If the secondary verbal group is passive, the meaning of the categories of phase is unaffected; but there is an effect on the interpretation of conative forms.