There's no reason you can't do both. Dual it at a lower level for BG1, but import the level 1 character when you start BG2 to dual at a higher level (9, 12, or 13 are all good choices. 9 was considered ideal mostly because of the SoA experience cap. If BG2:EE has ToB content the way that BG:EE has TotSC content, then going to 13 with the Kensai before dualing will net you a more powerful character in the end.
I'm not sure if this is popular but I disagree that dualing at 9 is best. I always dual a Kensai to Mage at 13.
That being said. In BG1 I would dual your Kensai at 7 and be a Kensai7/Mage8. I would either Shadowkeeper them to fix them for BG2, or save your game before you dual and finish it again with the pure Kensai.
Unless you're running a really small group (3 or 4 tops), I highly discourage dualclassing at 13 - mages in particular since they level slow. I tried DCing from Lvl13 Fighter -> Thief once in a 6-man group and HIGHLY regretted it. Keep in mind that a Lvl 13/14 Kensage would take 2.75M xp (almost the xp cap of SoA) and you'll probably only get around 3.5-4M total in SoA if you do absolutely everything with a full group.
You'll end up running a VERY large chunk (~75%) of SoA as a pure class, instead of enjoying the fun parts of running a hybrid class. While waiting to regain your first class, you'll probably feel very weak and useless compared to your party members because you're 1.25M xp behind. On top of that, you'll always be a vastly inferior caster in spell level access, spell power, and number of casts. Not only that, but you'll have worse THAC0 when under Tenser's Transformation because your Mage level is much lower.
You'll have to ask yourself: is the extra +1 dmg, slightly better unbuffed THAC0, and 0.5 APR worth it?
As for your original question, I would either DC at 7 if you REALLY want to to play a F/M in BG1 (you would probably hit it around the end of the game if you run a full group though) or wait and DC at 9 in the beginning of BG2. You'll get an extra weapon pip, more HP, +1 dmg, and better unbuffed THAC0 if you wait, as well as keep all of the stat tome upgrades.
Or like Ksarfeks said, you could always cheat a little with Shadowkeeper, if that's what you prefer.
Ugh, I've also tried playing kensai/mage dualled at lvl 13 in BG2 - that was a huge waste of time and made me switch to multiclass figher/mage ever since, which, imho, is much better than dual class and less complicated where metagaming is concerned.
I'm not sure if this is popular but I disagree that dualing at 9 is best. I always dual a Kensai to Mage at 13.
That being said. In BG1 I would dual your Kensai at 7 and be a Kensai7/Mage8. I would either Shadowkeeper them to fix them for BG2, or save your game before you dual and finish it again with the pure Kensai.
Dualing at lvl 13 might be the most optimal solution from powergaming POV, but it isn't anywhere near the most fun one from gameplay stand point. Dualing that late will cause you to not be able use the actual benefits of dualing for the vast majority of BG2. Instead you will be running around as gimp mage.
PS. Before someone says it I know there are "creative" ways to level up extremely fast.
Does creative count as memorizing tons of spells?'
I always am able to get to 13 Kensai / 14 Mage before Chapter 2 ends so I have the benefits of dualing for most of BG2.
That being said I only ever play a Kensai / Mage solo, they are way too powerful to make playing them in a group any fun, the rest of the party would just sit there.
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But of course you can always adjust it in Shadowskeeper later on.
That being said. In BG1 I would dual your Kensai at 7 and be a Kensai7/Mage8. I would either Shadowkeeper them to fix them for BG2, or save your game before you dual and finish it again with the pure Kensai.
You'll end up running a VERY large chunk (~75%) of SoA as a pure class, instead of enjoying the fun parts of running a hybrid class. While waiting to regain your first class, you'll probably feel very weak and useless compared to your party members because you're 1.25M xp behind. On top of that, you'll always be a vastly inferior caster in spell level access, spell power, and number of casts. Not only that, but you'll have worse THAC0 when under Tenser's Transformation because your Mage level is much lower.
You'll have to ask yourself: is the extra +1 dmg, slightly better unbuffed THAC0, and 0.5 APR worth it?
As for your original question, I would either DC at 7 if you REALLY want to to play a F/M in BG1 (you would probably hit it around the end of the game if you run a full group though) or wait and DC at 9 in the beginning of BG2. You'll get an extra weapon pip, more HP, +1 dmg, and better unbuffed THAC0 if you wait, as well as keep all of the stat tome upgrades.
Or like Ksarfeks said, you could always cheat a little with Shadowkeeper, if that's what you prefer.
PS. Before someone says it I know there are "creative" ways to level up extremely fast.
Does creative count as memorizing tons of spells?'
I always am able to get to 13 Kensai / 14 Mage before Chapter 2 ends so I have the benefits of dualing for most of BG2.
That being said I only ever play a Kensai / Mage solo, they are way too powerful to make playing them in a group any fun, the rest of the party would just sit there.