Last night, Doug Cole did a very thorough analysis of Rapid Strike vs. Dual-Weapon Attack, trying to figure out a fair way to merge the two and make DWA a special case of RS.
It's well worth reading, and with tweaks well worth implementing.
Naturally, I have a simpler solution.
A while back, when GURPS Martial Arts wasn't even on the drawing board yet, my players and I tried the following rule, which I put up in the comments. I've edited it for clarity. Where this version contradicts the comment, this version takes precedence. I wrote quickly there, but took my time here.
Dual-Weapon Attack is a special form of Rapid Strike (see p. B370). You may use this if you have two ready weapons or unarmed strikes. If you direct both attacks against the same defender, that defender gets a -1 to all Active Defenses against the attacks, even if one of the attacks misses. If you direct both attacks against the same defender, however, you may not make either half of the Dual-Weapon Attack into a feint - splitting your foe's attention works against this!
And that's it. That works with all of the existing rules options for 3+ attacks, multiple defenses, Dual-Weapon Defense, etc. It just nixes DWA as its own thing. The benefit of a DWAs are small but can be significant - the built in (and free!) level of Deceptive Attack, the fact that if you drop or break a weapon or critically miss with one attack, the other proceeds normally (they're simultaneous), for example. You really need Ambidexterity or Off-Hand Weapon Training to make it work, generally, which I don't think is even slightly bad.
How does this work in actual play? It was fine in playtest for our "Universal Combat System" stripped-down system. I still use this in my DF game. You won't notice this, because very few monsters and so far no PCs use two-weapon attacks. The rare times someone has done a shield bash-and-strike I've run it this way.
I don't have time today to write up DWA as a technique in this fashion using the Technique Design System, but it would be based on RS (so -6), give -1 to all defenses (-4), require 2 ready weapons (+1), only gets the defense penalty vs. one target (+1), can't be split into a Feint/Attack vs. the same target (+1) for a -7. I'd have to think if there is another +1 to be had from other limitations. I'd personally require you to pick a single weapon and single strike type (swing, thrust, whatever), like Targeted Attack, and give styles with access to this via a Perk the usual limitations they have already in their style writeups.
If there is a difference between Doug's rule and mine, it's that his is attempting to be mathematically consistent with the rules at written. My is attempting to be simple, yet largely consistent with the intention of the rules as written. I don't really care so much if you end up with different numbers than before, as long as there are less special cases and no great harm is done dropping one.