Glad you figured out the original issue. It depends what you want to do. Yes, you could use "on_new" or "on_load". The benefit of using on_activated is it's easy to determine what the last two files are. Though, you could argue on_new/on_load lets you manage it the same way. That is, you just need to know what the last two files you opened are. I'm relatively certain that on_load and on_new will be called, regardless of how they are opened. However, whereas on_activated is only called once, these listeners will only be called once. Well maybe on_load will be called more often if there are modifications outside of ST, I'm not sure. Perhaps you could put some logic in to store recent "on_activated" entries, in addition to "on_load/on_new" entries. Then, when you run the command, you can choose.
An enhancement you might consider making (if you are trying to learn more about the API or get more experience doing some Python) is adding some quick panel prompts. For example, you could present a quick panel with 2 entries. The first being an "ok" selection along with the current files. The second would lead to a secondary prompt so you may select alternate files. This would give you some indication of the actual files being used, rather than having to remember what the last files you worked with were.
As a final note, I believe this plugin would fail if one of the activated files didn't exist on disk (it wouldn't have a file name). Well, at least the version that came up from my google search.
I meant to mention this before, but having no response can mean multiple things. It is possible that no one looked at it. It is also possible that people are unsure on how to answer. As always, try to give as much detail as possible. You may also want to create a post on stackoverflow. That has the benefit of having notifications. Though I would guess that fewer ST users browse that tag compared to the users on the forum. SO does have the benefit of giving some sort of notification when a response has been made. Up to you though.