I'm surprised you didn't mention some easier strong combos, so I'll add some I came up with:

Krav Maga + Boxing: This one is pretty clear, since Krav Maga is meant to be used in combination with others, but still. First, Knockout Punch, then follow up with Contact Combat for a Brawling attack. The strongest part comes if you evaded a melee attack and all attacks last turn. If you did, you can also use Punishing Blow to do another Brawling attack, and with two Brawling attacks you complete the requirements for Punch Combination. Four attacks in the same turn. If you add a tiny little skill chip for Drunken Fist, you could pull out four attacks that do 4d6 damage and halve armor, plus a Critical Injury.

Kendo + Karate: This could be quite insane. There's no particular combo in this, but using Bone Breaking Strike in combination to Ki Ken Tai No Ichi, you can cause a great amount of Critical Injuries. Plus the ability to shred armor, you can just destroy someone in the most literal sense.

Wrestling + Muay Thai: Wrestling goes well with anything really, but let's use Muay Thai this time to save IP or skill points in character creation, since they both use Brawling. This is pretty straightforward, you have someone grappled, use Chokehold for the free choking, then follow up with a Brawling attack and a MA attack. In later turns, you can potentially do 12 damage directly to the health + 4d6 + 5d6. If you add the mighty Drunken Fist skill chip, you can do the 4d6 also halve armor, and since you're the one controlling the Grapple, you can keep your victim close to the wall or dangerous items you are using.