When dealing damage for the Two-Weapon Rend, does the Swashbuckler add it's Insightful Strike damage bonus to the rend damage if the weapon is a Finesse Weapon?
He certainly adds his Int to each of the two weapons’ damage before Rend even comes into play.
However, Two-weapon Rend is specifically “extra damage,” not another damage roll. Two-weapon Rend is a little vague about which damage roll this “extra damage” should be added to, but given that it uses the offhand weapon for determining overcoming DR, I feel it’s probably the offhand weapon’s damage roll. Thus your offhand weapon’s damage roll would be its usual damage roll, plus Int for Insightful Strike, plus the damage (1d6+1½Str) from Two-weapon Rend.
That said, Two-weapon Rend is a very weak feat, so I for one would be unopposed to the idea of applying Insightful Strike to it. In fact, I’d generally want it to do far more than that to be something appropriate for a 11th-level (or, more realistically, 15th-level on a ¾ BAB class) character.