[Riftmaker]

I'm just curious...

How could your K/M have enough proficiency slots to have 3 in Dual Weapon, 5 in Katana and at least 2 in Staff? The most you could have at level 7 and level 8 mage is:

3 In Dual Weapon
4 in Katana
1 In Staff.

Personally, I think you should just give Aerie the staff, or let your K/M only switch to it when absolutely necesarry.

Now, onto your spellcasting dilemma.

Main Mage: Aerie

Buffer Mage: K/M

Replace Jan with Yoshi if you're unwillling to waste spell scrolls on him and are just gonna replace him with Imoen anyway. When you get Imoen, she will more than likely become your main mage, thus letting Aerie focus more on cleric spells.

The K/M should be the buffer mage for this simple reason: If he's going to be fighting all the time, why waste the effort to pull him back from combat to cast a spell and then throw him back into combat afterwards? Simpler to just have him cast all his buffer spells before combat, and store replacements in sequencers. contingencies and triggers.

Give the Robe of Vecna to Aerie or Imoen when they get really high level mage spells...they'll make more use of it than your K/M would.