I just sent this to a player who had this very fair question and asked me via PM, so I will lay only my response out here for you all as well. Also notifs are messed up right now and I am not sure when they will get my reply via PM:

For option two, you need two threads, but however of those three options you want to combine in a combo would be up to the player. If that makes sense. So, here is my play and I call it: A Comedy of AC in Four Acts.

Say I had four threads over two months for Wanda and it's now time for February's AC. Thread one I started in December, and I tagged it four times in early December and five times in late January (and maybe I did or didn't tag it four more times in early February - either way works for this example, so choose your own adventure here), because I dunno, RL got crazy or I lost brain power for a minute. Thread one I can still technically use for February AC.

Thread two I started in early February and tagged it twelve times that day and then I dropped it or it wrapped. I can use thread two for AC, sweet, I have my two AC threads.

But wait, there's more, like an infomercial! Maybe thread two was NSFW and I am a shy bean about using NSFW threads for AC. I'm not, but let's pretend I am. So, I still need a second thread. Hrm.

Thread three I started in November and tagged it twenty times in November and five times in early December. I can't use that for February's AC since it was from a post three months ago, and now I'm like 'aw shit I guess I have to use my filthy smut for AC'. But I find a fourth thread.

Thread four, my lifesaver. I started this safe for work thread in mid-December and tagged it eight times that month, four times in January, and four times in February. I can use it for February AC and my dirty smut is not on my AC form and I can pretend like I'm not smut trash for another day. But I also probably plurked that smut thread anyway so yeah.