Edit- Never mind. I can now access dual class in the character screen. But what's a good level to start dual classing?

Thanks.

Depends. From what class are you dual classing to what class? Are you playing through just the original game, or are you taking the character through HoW and into TotLM as well? Playing HoF mode?

A good level to dualclass? Depends on what you want to achieve I guess. And also what classes you are mixing.

Fighters typically dualclass at lvl 3, when they have 3 stars in a weapon. You can do it on higher levels as well, but I believe lvl 3 is most common for fighters.

A ranger / paladin aren't allowed to put 3 stars in a weapon, so if you use them to gain weapon / armor proficiency, dualclassing at lvl 2 might be a better option.

A thief dualclass can benefit from higher levels than the typical fighters. You could for example level him/her as a thief until you max out find traps + lockpick, and then dual him to something more useful. You would then play a part of the game without a thief, but gaining the required levels isn't really that hard late game. If you don't want the hassle dualclassing thieves, you could of course use a multiclass instead. Personally I never saw much point in bringing along a single class thief in Icewind Dale, so I went for a mage/thief.

Cool. I'll wait till level 3.

I've been spreading out my weapon proficiencies. Is that a bad idea? Now that I read these comments it makes sense to specialize different characters with different weapons... Oh well. I think I'll see how it goes. Live and learn.

It's difficult to plan ahead which characters uses what, because you might find that the one character you planned to use greatswords is Good aligned and your supaweapon is only for Evil dudes.

But other than that, I would favor Longswords first, then Axes and Hammers tie. Don't shoot yourself in the foot if you find out the trollkiller (must weapon) is a great sword you don't have specialized (just equip it when trolls are semidead), or you find some cool anti-race weapon (rest assured that when you find one, it means you'll never see one of those enemy types again!). I used 2 melees and 4 ranged (ranger was backup melee) on my first go btw.

From the manual:

PROFICIENCIES
Every character has trained in a weapon during their life; your weapons of
preference are listed on this screen, each one described in the text box on the
bottom of the screen. Your class usually limits what weapons you can use and
how skilled you can become with the weapons you have chosen (mages, for
example, don’t have a great weapon selection because they’ve been nose deep
in books most of their life). As long as their class permits use of the weapon,
a character can wield a weapon that they lack proficiency for, but they will
incur a penalty on all rolls with that weapon. While most classes only allow
you to spend one point in each weapon, fighters, paladins, and rangers are
allowed to spend more than one. Every point above one gives them a bonus
to their attack rolls, damage rolls, and number of attacks per round.
Note, however, that only fighters can assign more than 2 points to a given
proficiency. Rangers, paladins, and multi-classed fighters are all limited to
2 points. Dual class fighters may only add points above 2 if they are active
in the fighter class.

Notice that if you use a weapon with no proficiency, you get penalties on hit rolls. While having more than one star in a weapon grants you bonus to hit, damage and attacks. It might not sound that great, but believe me it is. A fighter with 5 stars in say longswords will be able to chop his way through enemies scarily fast.

On my second playthrough I decided to experiment with dual / multi classed characters, and focus on ranged fighting.

I ended up with this party:
Fighter 3 dual classed to druid with 3 stars in crossbows
Ranger 2 dual classed to cleric with 2 stars in missile weapons
Fighter 3 dual classed to mage with 3 stars in bows
All out fighter - 5 stars in swords, rest in axes
Thief / mage multiclass
Bard

Not sure if I remember all of them correctly, but it was something like that. Also when I started the game I chose to have first one, then two characters. I used the ones I planned on making into cleric and druid. It is a bit cheesy, but I did it like that to regain my fighter levels more quickly. During the first part of the game you get a few fed-ex quests in Easthaven, and completing those with only one or two characters allows them to level up a few times before you set out for Kuldahar. This allowed two of my characters to get a head start, and I avoided some of the hassle it would have been to dual class three characters at once while doing quests in the Vale of Shadows.

It took a bit of planning to pull this off, and I know it's not for everybody. But I'm a bit weird and like to experiment with different party compositions. Overall I believe that party to be one of the strongest I have created, and also one of the easiest to play with casually after I got it started. It's packed with spellcasters, and all party members are proficient with ranged weapons. The fighter is the most obvious choice for a melee tank, but also the cleric and druid can perform well if needed.

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Sorry about the long post, just got sidetracked a bit when stating that focusing your stars in one weapon definitely is the way to go for the fighter classes.