I have seen this issue in the past, also for a dermatological study. We did not find a final answer, also as the study was discontinued.
The issue again shows one of the weaknesses of the current SDTM model. The latter assumes that a subject can never obtain 2 different investigational treatments in the same period of time in parallel. As I argued in my presentation at the CDISC Interchange in London, the SDTM model is on a dead track. For example, we see more and more studies that have no visits, but the SDTM model requires us to submit VISITNUM. How can we do that?
One of the ideas that we had for the issue you describe here was to make TAETORD of data type “character” in both TA and SE and then assign 2A and 2B for the elements. Whether this would be acceptable to the FDA we don’t know, as the study did not make it to the submission state.
As this is not a Pinnacle21 question, I would recommend to also submit your question to the “CDISC - SDTM Experts” forum on LinkedIn. There are people there with much more SDTM experience than I have.