Tempest 3: +5% double hit, main hand only.
Other Ranger Goodies: +2 dmg vs FE
Kensai 1: +1 Hit/Damage, +2 seeker stacking
Other Fighter Goodies: +2 dmg (Weapon Spec), +2 Str (Enhancements)
5% Double Hit
I'm going to try to explain this in abstract terms, as different weapons will change the gross DPS. In any case, though, we're comparing TWF builds, so the weapons should always be the same for comparison purposes. Tempest 2 gets 100% offhand proc. At a 10 strength (unrealistic, but hang on), offhand damage is the same as main hand damage. As strength increases, the balance tips in favor of main hand, as it gets the full strength bonus to damage and offhand only gets half. At 40 Strength, the Main hand is getting +15 and the offhand +7. Here would be the average non-crit damage of a mineral 2 khopesh (for argument sake) without effects: 5.5 base + 5 enhancement + 5 PA + 7 songs + 10 Fav En + 15 strength + 2 shintao set = 49.5. Off-hand would be 42.5. These numbers are only meant to be approximate, but the ratio there is: the main hand accounts for 53% of your DPS.
53% * 1.05 (double hit) + 47% = 103.5%, for a net DPS increase of 3.5%. I don't claim this to be gospel, but it should be fairly representative.
Kensai 1
(Base damage math above)
Average Damage Main Hand (-2 Fav En, +2 Spec, +1 Kensai, +1 higher Str): +2
Off Hand: +2
(2/49.5) * 0.5 + (2/42.5) * 0.5 = 0.020 + 0.0235 = 4.35%
And since Kensai will have slightly higher base damage and the +2 seeker, factoring crits will push Kensai even further ahead. I know, I'm splitting hairs here to debate over 0.8%. Versus non-favored enemies, though, the Kensai splash gets even better since it adds to base damage.
(2/39.5) * 0.5 + (2/32.5) * 0.5 = 0.035 + 0.03 = 5.5%
I will personally be LRing my Ranger into Ranger 13 / Fighter 7, I think (Ranger 13 and Fighter 7 are both loaded with good enhancements).