The correct answer in both scenarios is you place monsters on ALL gates (you still follow the rule that no other gate can get more monsters than the surging one). BUT, if the monster limit is reached, the monsters appearing through gates in Arkham go to the Outskirts; monsters appearing through gates in Dunwich are not counted against the monster limit, so they appear normally.
Before you ask the next logical question, if some monster has to move from Dunwich to Arkham (flyers, Hounds, Milk of Shub Niggurath) but the monster limit is reached, all monsters exceeding the limit go to the Outskirts.
So if a surge occurs in Arkham the monsters are distributed among Arkaham gates, or they go to the Outskirts if the limit is reached. If a surge occurs in Dunwich the monsters are only distributed among the Dunwich gates. Got it.
EDIT: Just read your edit and realised that I still don't get it. But I think I do now.
Lee, Tox didn't say that. He said a very different (and very important) thing. When you have to resolve a monster surge, you follow this order of actions:
a) determination of the number of monsters. As usual, you count the number of investigators playing and the number of gates in Arkham, which means here "Arkham + Dunwich" (remember "In Arkham = on the main board & all the expansion boards in play), see whichever is the greater: this is the number of monsters to be spawn. So, 4 investigators, 5 gates = 5 monsters (regardless of where the gates are, we could have 5 gates in Dunwich, 5 on the core board, or 2 here and 3 there)
b) you start resolving the surge as normal, hence by placing the first monster on the surging gate and then spawning monsters according to the order you decided before drawing them from the cup
c) a monster to be placed on a Dunwich gate is *always* placed there: there is no monster limit for expansion boards
d) a monster to be placed on the core Arkham board is subject to the monster limit rules: before spawning the monster on the gate, you count the monsters in play (only on the core board and in the Sky): if you haven't reached the monster limit yet, you spawn the monster on the gate, otherwise you put it in the Outskirts (rules for Outskirts limit still apply, clearly)
e) you keep on repeating until you've placed all monsters from the surge on their respective gates (or in the Outskirts if you have to place some on the main board but you have reached the Monster Limit)
that Flyers may move from Dunwich to the Sky: in this case, you check again the monster limit: if you've already reached it, the Flyers move from Dunwich to the Outskirts
Hope this clarifies any remaining doubts you may have. Otherwise, ask