Multi-classing is one of the things that I am still considering adding - but as an option, in Libram Novus, not as part of the core rules.
If I did add in multi-classing, it would likely be through the acquisition of a Talent that would maybe cost 5 points. Once taken, your skill costs would be averaged (like suggested in the Cross-class column above). The Multi-class talent would also make the Class talents available for purchase by the character (i.e. it makes them Trainable for that character, even if they were not Trainable before) - Thus a Fighter who multi-classes into Archer could buy the "Archery" talent, he would not gain it automatically....
At least that is/was the plan...
Your suggestion is intriguing, but I would have to see some examples of it before I settled on a decision about it. Limiting multi-classing to only 2 classes may be a good solution as well, and might be useful even for the original method I had planned. Hmm.... your suggestion would make multi-classing more attractive in the short term, especially if combined with the "must buy the Special Abilities" separately rules.....