No Andy, I don't have any idea, I only knew about the NEI Data Dumps because I used that feature to output all the block Item ID's for something I was doing. I'm really not an expert on the inner working of mods and minecraft, but I'm sure someone will eventually show up here to help answer that. Forum seems very slow today, but it is the weekend and Valentine's and all that stuff... It probably depends on the two mods in question that are conflicting, and how their configuration files work.
Here's how I would handle it, I'm just taking a wild guess, but I would probably install one mod without installing the other conflicting mod, then run the Biomes data dump. Then I would reverse it, install the other mod and don't install the first one, and run the Data Dump again. That should give the full listing of all the Biomes from each mod (maybe)? Good luck with it. What you're going thru is exactly why I generally just stick to the modpacks as the FTB team has set them up, and I don't mess around with it. It's great that you're trying, I'm not saying you shouldn't, but for me I hate dealing with headaches so I just play the packs "as-is", for better or for worse. But that's just me, other people here seem to never play any modpack "as-is", so I'm sure someone like rhn or Pyure or Mac will show up here to assist (eventually).