Elaine,

We have the ADaM gurus in our user group ;-), so you should be able to get a clear answer but I will take a shot.  This is based on our experience implementing in the CDISC Pilot. I probably have more questions for you then answers but here it goes.

The generic answer is 'however you want to do it' as the the model is flexible.  However, the real question is how would you plan to use those variables.  If you have an analysis that needs the treatment for both periods (even though they are the same) then include both.  If you are not going to use them then I suggest not including it and just having one treatment.  The other question is:  Do you plan to also collect the period date information within ADSL - TRT1STDT, TRT2STDT, etc.  If you are collecting this information within ADSL then I would suggest including TRT2P so you have consistency across your variables.

My last question is are you using the new (Draft) IG because I believe, from the conversations in the Pilot, that there are some subtle differences in how the TRT variables are interpretated.  I would follow the IG 2.1 guide.

Disclaimer:  This is only from my one experience and I would recommend waiting until one of the ADaM team members who are part of our group chime in.

Chris