How many threads do I need to earn or upgrade a trade?

Piggy-backing off this question, if you have a thread that encompasses the use of more than 1 trade (i.e. Barky and Woofy patrol the borders, chase away Trespassy, and then the thread turns into Barky giving Woofy relationship advice), could you apply it to both trades?

I think it's one trade/specialty per thread, otherwise people could use one thread for three different trades/specialties and really cut corners. Another admin can correct me if I'm wrong but I think this way keeps it both 'simpler' and more challenging.