St. Thomas scripsit: “considerandum est quod in quolibet novem generum accidentis est duo considere.”
Why didn’t he use “sunt”? Isn’t “duo” the subject?
"It should be considered that…there are two things to consider."
It should be considerare. I consider that duo considerare is to be considered the subject. 
Yes you are right. I mistyped it.
I know that an infinitive can be a subject but how does that work with duo? Even if considerare duo is the subject, shouldn’t it still be sunt?
No, because it is the action (duo considerare) that is the subject, not duo.
in the abstract it makes sense to say considerare is the subject and so would require a sg. verb, but what sense does it make in this case with est? Do we just not speak this way in English so we just have to say: “there are two things to consider” even though literally it says: “there is to consider two things”?