If this boss is getting in your way of getting your work done, then you have to get her out of your way as soon as possible.
I understand that you need to be as polite as possible because she really is one of the bosses and it doesn't reflect well if you are rude, However, It doesn't reflect well on you if you are lagging, no matter who or what is getting ur work done on time.
You have some options. Make sure you have a name of (someone who is leader on team B) for the boss. When approached, say you need to speak to Karen Hartly, she will have all the answers you need. If she insists, say your best bet is to go straight to the source for all those questions, as i'm not a part of that team
Or
Take the boss over to Karen Hartly, tell her Karen Hartly, good morning this is so and so, she has a few questions about your data. Would you be so kind to speak with her. Karen is not going to say no in front of the boss. So, there you go.
Or
You can be politely forward with her. Tell her, I know the data format is difficult to understand (say that so you don't insult her intelligence) can we set up a time to meet so we can go over this? I'm pressed for time as of now and I really need to get this work done. Is this ok?
Or
You can go to your boss and nicely explain that so and so is having a really hard time understanding the data format. I explained my best to help her out but its still difficult. Could you have perhaps Karen Hartly explain, as it is her team. I know how important it is to get this work done in a timely fashion.
see what your boss says.