Oversized TWF feat, Complete Adventurer. Treats one-handed weapons as light in the offhand. Your Large kukris are one-handed weapons when wielded by a medium char, so your TWF penalty is -2/-2. You no longer have any monkey business, so it becomes a straightforward TWF calculation.
No Tempest
Mainhand: BAB - 2(twf) + 4(enh bonus) + str
Offhand: BAB - 2(twf) + 4(enh bonus) + str
Tempest 2
Mainhand: BAB - 1(twf) + 4(enh bonus) + str
Offhand: BAB - 1(twf) + 4(enh bonus) + str
Tempest 4
Mainhand: BAB + 4(enh bonus) + str
Offhand: BAB + 4(enh bonus) + str
Er, after rereading the weapon size entry you'll have a -2 penalty when using a large kukri, something that monkey grip doesn't reduce. I've never understood that, really - the only difference between a shortsword, longsword, and greatsword are the size category. A large shortsword is a medium longsword - they have identical stats. In your case, you've got Kukri and Scimitar.