You know, that's an excellent question.
I guess you'd have to first consider the number of hours in a day, and how much time you could actually dedicate to each profession to make it/either one profitable. Don't forget, the PCs/heroes also have to find some time in there to adventure as well.

A Wealth bonus increase may or may not be applicable since you can't be two places at once. EDIT: Oops - my suggestion here - maybe average the two bonuses together.
Next, you'd have to consider why the character has two professions, and which one is the dominant meal ticket. Using your example, car mechanic/writer (or my personal favorite waiter/actor): Is he/she just starting out as a writer (or actor) looking for that "big break" in their artistic field? Then perhaps their Wealth bonus would reflect that of the mechanic or waiter, with a +1 added for the wee bit of extra income they initially earn (doing a play in the evenings "Off Off Broadway, so Off Off That It's Actually Dinner Theater in Hoboken", e.g.). If the car mechanic has a few books on the
New York Times Best Seller List about auto repair, however, then the Wealth bonus would be higher, that of a successful writer. Another example that comes to mind is the physician who is also national news "health beat" anchor - he/she may have a successful practice, but they earn extra income for the reporting work they do.
Of course, characters like Buckaroo Bonzai should be the exception, unless you want that kind of game
This should be a good thread
