Understanding your disdain for 'skill for each combo' so the best rule I allowed for it in our campaigns was the Highest Weapon Skill Cost rule to cost the TWC skill out in the first place followed by the Add the Weapon Fumble Ranges Together (for LH+RH) and this was however many ranks expressed as a percentage of the OB one could use for this TW Combo.
I liked it because at lower levels, it made sure most effective combos would tend to be with "lighter" weapons with smaller fumble ranges.
If not, TWC would show its reduced OBs when not using 1H weapons singley.
~~PC wants to learn Right Hand(RH)-SS/Left Hand(LH)-SS.~~
+40ShortSword(SS) fumbles from 1-2. SO TWC needs 4 total ranks to be able to use 100% of OB.
Since highest weapon skill cost is a single digit for every class, it will take 4 levels before full OB use is obtained - until then, every rank gained in TWC allows for 1/4=25% of the SS Combo being learned.
If no further actual weapon development in ShortSword is done, every TWC rank developed allows +10 more OB to be used in both attacks/swings.
If you don't want to restrict this, you could say any combo of weapon fumble ranges up to 4 could be used, so BroadSword Fumbles on 1-3/Dagger 1-1, Mace 1-2/Mace 1-2, etc.
Of course, the restriction is how well developed all other weapons comboed up are.
To further modify this, unless (N)PC is Ambidextrous, LH OB gets modified by whatever you deem, usually -20+, so above SS example would elicit full TWC rank OB of +40RH-SS/+20LH-SS.
No charts to look up skill costs - besides character sheet itself - and nothing besides calculating less than full TWC rank OB%s if total Fumble ranges are greater than TWC ranks to offset them.