Ah, missed it, it's there all right.
The tweaks can be found in preferences->tweaks. Basically, what you do if you want to use that tweak is first to copy and paste it down from the upper tweaks window to the lower one (the upper field contains all available tweaks, the lower one contains the ones that you are actually using). Then, change the tweak to whatever value you want to use according to the documentation (the stuff preceded by hash signs '#'). For your case, putting
Code:
sort_columns_at_startup = [('authors',0),('title',0)]
into the lower tweaks field sets your library to be sorted primarily by authors and secondarily by title at startup.
Note that when adding tweaks to the lower, "in-effect" field, you don't necessarily need to include the tweaks comments.