Dual-classable NPCs

[Lemernis]

Dual-class rules:

1) the character must be human
2) the primary ability score(s) in the original class must be at least 15
3) the primary ability score(s) in the new class must be at least 17.
4) after dual-classing the original class skills are deactivated until the character reaches one level higher than the original class; then both sets of class skills are active

Therefore, the BG1/EE NPCs that may be dual-classed are:

Branwen
Dual from Cleric to Thief using the Manual of Quickness of Action (Dexterity +1, located at the Thieves' Guild in the city of Baldur's Gate).

Dynaheir
Dual from Invoker to Cleric using two Tomes of Understanding (Wisdom +1; locations: the 3rd Floor of Durlag's Tower, from the Lady's House in West Baldur's Gate (also needed for a subquest), and in a coffin in the catacombs beneath Candlekeep).

Imoen
Dual from Thief to Mage. Her ability scores allow this without having to use ability-boosting tomes.

Safana
Dual from Thief to Mage using the Tome of Clear Thought (Intelligence + 1, located in Ramazith's Tower in North Baldur's Gate).

Shar-Teel
Dual from Fighter to Thief using the Manual of Quickness of Action (Dexterity +1, located at the Thieves' Guild in the city of Baldur's Gate).

Xzar
Dual from Necromancer to

Cleric using one Tome of Understanding (Wisdom +1; locations: the 3rd Floor of Durlag's Tower, from the Lady's House in West Baldur's Gate (also needed for a subquest), and in a coffin in the catacombs beneath Candlekeep)

or

Thief using the Manual of Quickness of Action (Dexterity +1, located at the Thieves' Guild in the city of Baldur's Gate).

Ability score boosting Tomes and Manuals locations are listed here.

[Lemernis]

The question of when to dual-class depends on whether one prefers a more optimized build versus having both sets of class skills sooner rather than later.