Hi I tried hand at excel first time so I made quick list of possible shield mod combinations 
Im not 100% sure I got all possible shield mods but I calculated with simple math which might be horribly wrong
that theres 45204 possible max damage shield combos that drop today. I put screencapture of my messy excel I doubt no one can make sense of
There are probably mistakes so correct me and I fix 
Edit 24.4 added +56hex mod
Condition mods cant be nonmax so that 19% condition mod is false.
also missing vs 9-10ar vs Orges
The requirement is imo not worthy "mod combination" so i would remove that imo since its not rly mod, otherwise you could add diff armors too..
Im not rly sure which combos u count as possible tho, which are primary and which secondary and which both. ? since like Health mods you cant have many in same shields but some secondary mods (1/20 + dmg reduction example, or 1/20 + armor) you can have in same shield, but example two dmg reductions in one shield isnt possible Edit ah ya now got it for this part
We did these similar calculations for shields and offhands awhile back, also difference between tyria/cantha since some creature mods drop both, some only either,
Offhands have even more possible mod combinations then shields, therefor are even rare with good matching mods.
Excel is my 2nd language (maybe even first hehe), so this makes complete sense to me. I don't see any mods immediately that is missing and the calculations done to avoid duplicating combos looks good.
One thing to note, I am assuming your intent is to show the number of Dual mod possibilities. With the data you already have, you could also determine the number of different single modded shields that would drop that would give you a grand total of OS shield possibilities. Not that you have to do that, just a spin off idea if you were interested.
Great job!!
Dazed condition is a possible drop, here is one from my collection
As far as dual mod drop rate, here is some data from some of the things that I track specifically from chest drops:
Out of 1073 OS chests (combo of NM and HM), 48 items (wands, focus, shields) were dual modded (4.47%)
8 were dual modded shields (purple or gold) (0.74%)
3 were dual modded gold shields (0.28%)
Total of 45 shields from the 1073 chests, 8/45 dual mod (17.78%).
Chevy
