Some things to keep in mind..
For RMFRP at least, the above 120 in Shield Bash and 120 Sword OB example, would still be limited by the TWC skill. The OB for either weapon in TWC is limited by the lesser OB (in this case they are the same, 120) and can't be higher than the skill in TWC. (Although magic and off-hand adjustments may apply.) So, weapon OB can't be higher than TWC skill.
Since the OB (before the OB/DB split) is limited by the TWC skill bonus, it will limit DB for the character.
If you somehow had 120 in Sword, 120 in Shield Bash it would be very difficult (but perhaps not impossible; players are a crafty bunch.) to have 120 in TWC. The OB is for the Two-Weapons-you-use-in-Combat, skill.