Its not allowed in any official rules.
As for house rules I dislike the idea of having a talent that only acts as an xp sink, i.e. buy this talent, it does nothing for you right now (as that is no fun). Then buy your secondary class.
I also dislike the idea of having to pay a premium like x3 xp to increase a multiclass, cause in the end you might want to become better in your new profession then your old, and then you might be better off just killing yourself and rolling a new character (not fun).
Finally there is the idea that multiclassing bars you from your old profession, and I don't really like that either.
So I allow player's to multi class freely as long as they have two levels in ALL the other professions starting skills.
Kind of like "sure you want to be a sorcerer, but then you got to learn the way of the sorcerer first".
It is also good because professions that are further from each other like "Druid + Sorcerer" is harder to accomplish, then "Rider + Champion" as they share less starting skills.
So far only one of my players have multiclassed. From Champion to "Champion + Druid (Healing)" because he wanted a more Paladin like feeling with lay-on-hand.
But in I have however also lowered the cost of level 2 skills from 10 xp down to 5 and that helped a lot (using Reforged Power house rules).