I've long thought that most warriors would have a shield rather than fight with two weapons - not thinking of superheroes, just the ordinary rank and file.
Getting 3 protection for a buckler, doubled up to 6 with the warrior's bonus, just doesn't compare to using a 2d6 +5 kukri though. It's a no-brainer to take the kukri unless you're determined to run with the ideal image instead of the best fighting mechanic ploy.
Leaving aside the ambidextrous, I'm inclined to take two stabs at this, bearing in mind the KISS rule:
1) Give a 50%, rounded down, bonus on the shield's normal protection to warriors per level eg a L1 warrior would get an extra point of protection and so the buckler would take 8 hits while a L2 warrior would get an extra three points so the buckler would take 12 hits, etc
2) Give offhand weapons only 50% effectiveness with dice and adds rounded down eg the L1 guy would not get 3 +5 I've factored in the warrior's dice bonus here) but 1d6 +2 while the L2 warrior would not get 4 +5 but 2 +2.
With these house rules, the L1 fighter would get a benefit of 8 from the buckler compared to 5.5 for the kukri with the L2 warrior getting 12 against 9.
That seems to work (at least for that shield v that dagger at those 2 levels)!