Normally FANTASTIC HEROES & WITCHERY doesn’t allow multi-classing as in earlier editions of the game. The reason being that multi-class characters are seen overpowered at equal XP when compared to single-class characters. Lets take an example: with the same amount of XP, a character could be an 8th level fighter, or could be a 7th level fighter-wizard. So you could be a slightly inferior fighter (i.e. one level lower), and with less hit-points too, but with the spellcasting ability of the 7th level wizard. If only firepower and survivability are considered, who will hesitate? Same remark when you compare the 7th level fighter-wizard to a simple 8th level wizard. For this reason, FANTASTIC HEROES & WITCHERY instead proposes split-class characters, for those who would want to pursue two careers rather than a single one. Then, as fighter-mages were much popular back in the days, and split-class fighter + wizard characters tend to be weak, the war-mage class was also introduced.
Nonetheless, there might still be room for multi-class characters as in earlier editions. That is: combine two classes. The character gets the better BtH and save progression of the two classes; all the class abilities of both classes; and for hit-points, half the sum of both classes’ hitdice (round fractions up). However, to prevent multi-class characters to be too powerful, they will be restricted to NPC classes, which are less powerful (have no special abilities) than adventuring core-classes. These NPC classes (which are detailed p.130-131 of the core rulebook, and are also reproduced here) are the: Fighting-man, Clergyman, Cultist, Magic-user, Professional, and Thug.