OK...
Then Dual Sai bonus + Item Bonus = Total OB
But, IIRC, dual weapons allow only one attack roll. So, using 2 +10 sais with dual weapon still only grants a net +10... not +20(+10 per sai) to OB.
I agree with above. If they were used as a duplicate bonus you could end up with 2 Sai +25 weapons and an OB bonus of +50.
A white alloy master work sai, for instance, would be +25 and 30x the base cost of 13sp. So for 78gp a player could buy 2 white alloy master work sais for a bonus of +50 to OB. To me, this would be to easy for a low level character to achieve.
Also, if the weapons have different bonuses, I'd add the bonuses togther, divide them by 2 and round it down. (So +15, Sai +10 Sai = 25. 25 divide by 2 rounded down = 12. OB bonus is +12 from weapons.)