Nope, that -5 is the same at the 2nd iterative attack on the on hand. You have that AND the -2 penalty.
BAB=+10
STR=+5
+1/+1 Quarterstaff (you need to enhance both ends of a double headed weapon separately).=+1
On hand, first attack=+10 (BAB) +5 (STR) +1 (wpn) -2 (TWF) = +14
On hand, second attack=+10 (BAB) +5 (STR) +1 (wpn) -2 (TWF) -5 (iterative attack)= +9
Off hand, first attack=+10 (BAB) +5 (STR) +1 (wpn) -2 (TWF) = +14
Off hand, second attack=+10 (BAB) +5 (STR) +1 (wpn) -2 (TWF) -5 (iterative attack)= +9
Notice that both 2nd attacks have both the -2 for two weapon fighting and -5 for 2nd iterative attack. But you don't count it twice like I think you were doing.
Good luck,
=Blue(23)