§3.2.4. Advisory Committee Meetings contains the following text:
Each Member organization should send one representative to each Advisory Committee Meeting. In exceptional circumstances (e.g., during a period of transition between representatives from an organization), the meeting Chair may allow a Member organization to send two representatives to a meeting.
This text predates the creation of Alternate AC reps, which are defined in §3.2.2. Participation in the Advisory Committee.
The quoted paragraph above currently only allows Member organizations to send two reps to an AC meeting "in exceptional circumstances." Now that Alternate AC reps are a regular, ordinary thing, I suspect we should, in general, allow Member organizations to send both their AC rep and their alternate, and not only "in exceptional circumstances."