Any assembled tandem made from any material should be clamped using the stoker's seat post when placed into a workstand. Unless you're working on the headset or have a reason to remove the front wheel and fork, just let the tandem rest on the front wheel. As for the seat post material, there's no need to swap out a lightweight aluminum or composite stoker's seat post when doing basic maintenance in the workstand if the front wheel or front of the tandem is supported.
If you do need to work on the front end of the tandem, just put a 5 gal bucket or step ladder under the front bottom bracket while still supporting the tandem by the stoker's seatpost.
About the only time I've ever used the captain's seat post to hold the tandem is when I'm working on a bare-bones frame....
