Well yes, that is a painm but that's the thing with all dual-classing, there will be a rough period where your guy will retty much be useless.
Here are a few tips that will make the transitional period easier for a kensai/age.
First, save up a buttload of money and buy as many spells you don't already know at once. Then, kick everyone out of the group and start scribing. You can ear a couple 100k's worth of exp this way. Beware that some npcs like Jaheira you can only kick out once, depending on your responses to their attempts to talk to you. Kicking them out a second time will cause them to leave you forever.
Second, time your dual-classing, I dual-classed my guy when Lord Firkraag was at "Near Death". Paused, dual classed, and then kicked everyone out or the group, then after I dual classed, I used a wand of frost to finish him off, 64000 exp.
By the time everyone walked up to me to ask me if I really wanted them to leave the group, it was too late. So I just accept them back in as if nothing has happened.
Third, kick people out when you're about to recieve a exp rewards from a quest, these are usually large chunks that means almost nothing when divided amongst 5 or 6 poeple, but alone, it's a major boost.
Finally, know when to start dual-classing. Most people will recommend starting the dual when your kensai reaches level 9.
Why? Because at level 9 you get a a new proficiency point.
And if you stopped at 9, you still have enough room in the exp cap to get to level 17 with the mage, which is the max level for a mage even if you're totally dedicated to a pure mage. So it's like you're playing a full fledged mage PLUS 9 levels of warrior abilities.
And it's really easy and quick to get to level 10 with a mage, so you have less downtime.
Personally I went with a level 12/15 kensai, Because I'm thinking ahead to Never Winter Knights when full 3rd edition rules are implemented, which means I will be able to go back and develop both my kensai skills and continue my mage skills further when I import this character. And as a level 15 mage, I still get level 7 spells, even though I'll be missing out on some nice level 8 spells like Simulacrum and Improved Mantle, to me it was still worth it to get that extra proficiency point at level 12.
Hope that helps, because to me, the kensai mage is truly the best class to go in terms of homicidal potential with pretty much no sacifice to the ablities and bonuses of either classes.
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