Or as I like to think of it - giving the swashbuckler two weapon fighting.
I've been looking at ways to get a swashbuckler to more attacks (outside of haste of course). The idea being at higher levels getting more attacks with precise strike is better than increasing the damage by a couple of points.
My first two thoughts were Magus and Monk.
Weapons that one can flurry with however are not that interesting, especially since we are looking for something with a high crit range. The one saving thought I had was Crusader's Flurry. So with a dip of Monk and Cleric you can flurry with a Rapier. Using FOB and if you can buff Divine Grace would mitigate the BAB loss.
Magus would seem pretty cool - require at least 2 for Spellstrike but I am thinking 3-4 levels of Magus would be better. Wand Wielder for an arcana so one does not have to deal with concentration checks. Using a Chill Touch wand(s) at lower levels, then possibly a wand of Frigid Touch if you want to sink in some gold (plus staggered is a nice debuff). Using your Arcane pool to enhance the weapon would help mitigate the BAB loss. Possibly go Kensai for Weapon Focus and getting the Wakizashi as a weapon.
The last one I thought about later was just Ninja with Ki Strike. Panche/Ki does synergize nicely with Cha as a stat.
Any of these worth it?