Xanterith said:I would read that as a -5 to the attack bonus of the 1st off hand attack, or +6 - 5 for +1. The it in the sentance refers back to the first part of the sentance, not to your BAB, at least to me anyway.
I believe the "it" in the sentence refers back to the weapon, as in "you can make a second attack with the off-hand weapon". You can then break the sentence by dropping the "albeit" which actually doesn't seem to make any sense (the word in that context) based on dictionary meanings, for which the common meaning is "even though". As I said if they added the text to state with an additional instead it translates well.
albeit
Which per my Britannica dictionary means:
: conceding the fact that : even though
On-line dictionary:
although; even if: a peaceful, albeit brief retirement.
Even though; although; notwithstanding: clear albeit cold weather.
But I agree the meaning (at least per common sense) is that it is with an additional -5 penalty.