Here is what we have decided on to make characteristic advancement not hog up an entire career, and allow more than one stat to progress over time.

- Fortune advancements can be placed on any characteristic, and will contribute to the cost of raising it to the next level

-Characteristic advancements can be spent across careers. For example, if a character wants to increase his Toughness from 4 to 5, he could invest 3 open career slots in his first career, and 2 in his next career. 

- Points invested on characteristic advancement act just like Fortune advancement, up to two white dice as that characteristic improves. So in the example above, as he invests in Toughness, his 4 will have 1 white die, then 2, and then hit 5 with no white dice when he invests the final point and  reaches Toughness 5.

- The career where the investment starts has to have the the characteristic as one of its primary characteristics. If the next career does not have it, the points will then go to non career slots and the player will have to spend the extra point to finish his advancement. 

Interesting, I did exactly the same :) You can find it in my house rules, it worked fine for me and my players, they liked it a lot.