Yes, it is easier to use TWF in Conan than in D&D.
But I don't think this is unbalancing, due to two simple factor:
1) Damage Reduction. The Armour penetration of all the weapons you can weild in your off hand without causing a penalty is very poor. Most won't penetrate even a leather jerkin unless the weilder is exceptionally strong.
Fundamentally, if you choose to make two lesser attacks instead of one stronger one, your opponent gets twice as much effect out of his armour.
2) Sheer Damage
Two-handed weapons in Conan are a lot more damaging than their D&D counterparts. 1 attack with a greatsword gives you a roll of 2D10, which is hard to beat with any pair of lighter weapons that wouldn't give you a penalty. And its Armour Penetration is significantly higher, too.
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Compare: Vs a Leather Jerkin, Strength 14. We could use a higher strength, but we could equally use better armour, so I thinks its a fair comparison. 1 Greatsword, or a War-sword and a Shortsword.
1 Attack with Greastword: Damage 2D10 +3 (strength) -2 (armour, penetrated)
Net Result: 2D10 +1
1 Attack with War Sword: Damage D12 +2 (Strength) -2 (armour, penetrated)
+ 1 Attack with Shortsword: Damage D8 +1 (Strength) -4 (Armour, unpenetrated)
Net result: D12+D8-3
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That ignores a lot of factors - critical hits, the minimum damage one hit can do, etc etc. But I think it is a fair comparison. After all, it also ignore the fact that once you have gone beyond one natural attack, you need to spend a feat on your fighting style that you could have spent on (eg) weapon specialisation.
If you include those factors, two weapon fighting probably has the edge if you are up against unarmoured, or lightly armoured opponents. Which is probably why Borderers favour it, since they spend most of their time battling scantily-clad savages.
But against anybody with any decent amount of armour, two-handed weapons almost certainly have the edge.