The standard method for adding info to cards is to export one deck to text, then re-import the info into the 2nd deck with "update duplicates" turned on. The downside to this method is that if you have any study data associated with the first deck it will be lost. But I don't think there will be a way to preserver that data regardless. You would also want to make sure that there are no duplicates within each deck, or at least allow for the possibility.
Anki Small Questions Thread
2017-04-17, 4:18 pm
2017-04-17, 5:08 pm
So if the info I'd like to add from Deck B is "ID" I need to make an "ID" field on Deck A? Is that it?
2017-04-17, 5:28 pm
Yes. And you would also need to make sure that the first field of deck A containes unique values that anki will use to determine which notes are duplicates.