I think this is the right approach. Calculations aside its not fair and it is misleading if your 72% dd xbow is not actually reducing damage by 72%. I think it should just cap out at the highest current DD (which is 72) regardless if you combine 2 weapons together or not. Note that a normal forge 52% dd and 52% dd would get you 78% DD.
In regards to dual wielding what we could also do is make them two separate sources of dmg reduction. With a probability to reduce on the offhand.
Lets say we got 2 static 50% dd weapons.
10k hit> 50%dd= 5k chance to reduce by the offhands dd (50%/2=25% dd) = 1250 dmg.
Then we can make this chance based on the tier of the dagger
1: 10%
2: 15%
3: 20%
4: 25%
5: 30%
6: 35%