Some possibilities:
1. Perhaps it might simply work with the present system, with both threads in the Disputes and Prosecutions forum.
People could then link to posts in the other thread. For example, in the advice thread someone could say an issue was being raised or discussed in the discussion thread. Or someone could tell the OP about, for example, a factual matter on law or procedure which had now been settled in the discussion thread. That could be tried out for a limited period.
2. If that seems to clog up the forum, then perhaps there might be a D+P Advice forum and a separate D+P Case Discussion forum.
3. Perhaps there could also be something like a "Disputes and Prosecutions: General Issues" forum.
This would be for clarifying issues that keep coming up, or seem likely to. It could end up as partially a reference work with sticky posts. Or some posts could end up contributing to the Fares and Ticketing Guide, or something similarly permanent, more focused on procedures for avoiding/sorting out trouble.
An advantage of a separate General Issues forum could be that it's much easier to see what's been discussed recently (or search for older issues). This might sometimes help people make good use of their time if it answers queries that otherwise would be brought up in other threads.