Don't hate me Swole Patrol... ;-P
those are cool suggestions but, IMO, that's a lot of added functionality for something that can be accomplished by making two attacks with an Attack Mod penalty for TWF. (primary and off-hand, light weapon?, TWF feat?).

With the All Attacks Ability Command button you can easily choose the TWF version of your attacks and you don't have to open the sheet.
That said, here's a thought on how this could be added to the sheet as a simple addition;
Add a TWF checkbox to attacks. If checked, two drop-down selectors are revealed to determine the total penalty for @{twf} for that attack. @{twf} would start as a base "-10" penalty.
Hand? Primary (+4) Primary w/feat (+6) Off-hand (+0) Off-hand w/feat (+6) | Light Weapon in off-hand? Yes or Unarmed Strike (+2/+2) No (+0/+0) |
These would all be repeating attributes within the attack
@{twf-toggle} possible values="1 or 0"
@{twf-hand} possible values="0, 4, or 6"
@{twf-lightwpn} possible values="0 or 2"
@{twf} value="(@{twf-toggle}( -10 + @{twf-hand} + @{twf-lightwpn} ))"
@{twf} would then be included in the total-attack calc.
This is not tested. Chris and Magik might have some ideas on TWF as well.