Then let's look at it from this angle: The character at some point, possibly in his childhood, learns of warriors who are whirling dancers of death called dervishes. Perhaps she witnessed one of their dances as part of a performance. (DM allows the dervish PrC.)
Seeing this beautiful, leathal dance, she is quite impressed by not only the way they combine the beautiful dance with dangerous weaponry, how they seem to wield a scimitar so effortlessly, as if it were a mere dagger. (Player reads up on the dervish, assuming he hasn't already, and likes the idea using scimitars as a light weapon.)
In an effort to possess the same skill with dancing, combat, and scimitars that dervishes have, the character spends a substantial amount of her free time learning to dance, tumble, and wield a scimitar as effortlessly as dervishes do. (Player gives the character ranks in perform (dance), tumble, and the feat Weapon Focus (scimitar), as well as the other feat requirements as the character advances in level.)
Having significantly more agillity than strength, the character, learns how to use weapons in a way to maximize the use of her agility in combat, knowing that when her training with the scimitar pays off, she'll be able to apply that same agility to it. (Player gives the character Weapon Finesse.)
Finally, after weeks/months/years of practicing and adventuring, she is able to become a whirling dancer of death, and is able to wield a scimitar as easily as she once did a rapier if not more so. (After finally getting his character's BAB up to +5, and getting the necessary XP to advance a level, the player gives his character a level of dervish.)
So yes, the character knows she'll be able to combine the two feats, just as she knows (a) that she has to have the feats Combat Expertise, Dodge, Mobility, and Weapon Focus with a slashing weapon in order to become a dervish, (b) that she has to have three ranks each in perform (dance) and tumble along with the feat requirements, (c) what the class features of dervish are to want to become one, and (d) that such a thing as a dervish even exists in the first place.