Clarifying combat math with Imp. 2 Weapon Fighting...

[Cygnia]

lvl 10 character fighting with 2 light weapons. Has TWF & ITWF as feats, BAB 10/5.

Now, I know the penalty for the 1st attack is -2 for each weapon. How exactly are the penalties calculated overall for the 2nd attack? Only -5 for the second light weapon or -7 or something else?

Math is not my strong suit

[sleepyphoenixx]

The -2 penalty from TWF is on top of every other penalty, including the -5 one on iterative attacks.
Which means it's at -7, same as your first iterative with your mainhand.

Your attack routine at BAB 6 with TWF and ITWF would be -2/-2/-7/-7.
At BAB 11 it becomes -2/-2/-7/-7/-12 or -2/-2/-7/-7/-12/-12 with GTWF.

[Kurald Galain]

Your attack routine at BAB 6 with TWF and ITWF would be -2/-2/-7/-7.

Yes, that.

[Remuko]

lvl 10 character fighting with 2 light weapons. Has TWF & ITWF as feats, BAB 10/5.

Now, I know the penalty for the 1st attack is -2 for each weapon. How exactly are the penalties calculated overall for the 2nd attack? Only -5 for the second light weapon or -7 or something else?

Math is not my strong suit

with BAB 10/5

you would have 8/3/8/3 as your 4 attacks

[Particle_Man]

Note that there are various ways to reduce the -2 penalty a bit (at least for some weapons) (prestige classes being the one off the top of my head, like Exotic weapon master or blood claw master). There are probably feats for this too but I can’t think of them right now.