Dim Vision and Terror don't stack, it's just a question of AI checks.
Basically, think of it this way. The monsters are just pixels on a screen. They have no brain or processing power, so they can't make any decisions. So how do they take actions, if they're just pixels? They "check" with the game. Basically, they're programmed to stop and ask for directions at certain intervals. For some monsters (like Zombies), these intervals are very far apart. That's how you can often sneak up and attack a Zombie without it noticing you until you hit it- if the computer's last command was "wander about aimlessly", and it hasn't reached a new AI check, it will have no way to recognize that you're there. When it reaches an AI check, the computer says "Oh no, there's an enemy in your awareness radius, go attack it!"
So anyway, the two most basic ways that AI checks are reached is at certain natural intervals (again, monsters like Zombies check infrequently, while Demon Imps check constantly), and when the monster can no longer perform whatever action it was performing in the first place (for instance, if you taunt a monster that has a ledge between you and them and no stairs up in the Bloody Foothills, it will quickly realize that it can't do what it was told to do, and will reach a new AI check). These aren't the only things that will force an AI check, though (for instance, I'm convinced that getting struck forces an AI check, because every time I sneak up on and hit a Zombie, he wakes up. And it's not just getting struck- the knockback from Leap forces an AI check on everything in its radius). Likewise, none of these are hard and fast rules. AI checks are sometimes touchy things, and are certainly not an exact science.
So, with that said, the reason why Terror and Dim Vision are "stacking" is because of how the skills work (with regards to AI check). Terror FORCES an automatic AI check. When the monster checks with the computer, the computer says "RUN AWAY!!!". Dim Vision *does not* force an AI check... but the next time the monster checks with the computer, the computer says "You're blind!" So when you cast Dim Vision on a terrorized monster, it's not stacking- it just doesn't go into effect until the monster reaches an AI check. This is true of any action the monster is performing. For instance, if a fallen (who goes a long time between AI checks, usually) decides to attack you, and you Dim Vision it, it can chase you all across the blood moor before it finally realizes it's blind.
