I can't seem to find the 2d12 to Large conversion anywhere in the charts.
Double Hackbut comes from Ultimate Combat.
While I don't have the Ultimate Combat book, yet, I'll take a quick look at the damage die table from core and the prd. Closest thing they have is 2d10 goes to 4d8, which is what I would have suggested originally. 4d10 wouldn't be too far of a stretch over 3d12, and they set the precedent. I'd go with 4d10. Enjoy double hackbutting something!
1d10 goes to 2d8. 2d10 goes to 4d8.
1d12 goes to 3d6. 2d12 should go to 6d6.
Also 2d12 is about the same as 4d6. 4d6 goes to 6d6.
4d10 and 6d6 have really similar average damage as well.
If upgrading further, say with enlarge person, 6d6 goes to 8d6, but what does 4d10 go to, 6d8? 8d6?
Well, I'd up it to 5d10 and then 6d10 (or 10d6). As a personal preference, I try to keep the number of actual die rolled to a minimum to speed up play. I can't recall off the top of my head for any official rules for damage over a certain size, but that's where common sense and the GM's judgment call comes in.
Switching to the mentioned d10s allows for a nicer mathematical ease of use and clean averages. Need to know the next size category of attack? Just add on the d10. If taking the example to its uppermost heights, I can see a d100 roll (or percentage dice if you don't have what I lovingly call the golf ball) being a reasonable resolution.