So short answer: No, it is not counter-productive.

First of all i think you have a good approach on draw-a-box, i liked how you said that you want to finnish it in a marathon. This is a very healthy approach.

In the end its all a matter of what situation you are in. If you were trying do to as many courses in a shortest period of time possible, that would be counter productive. This way you would try to gain as many informations and knowledge about drawing as you can, but things just dont work like that. In the end it all runs down to you having to draw a lot, courses will only point you into a certain direction or path so to say.

Draw-A-Box is not supposed to be some kind of prison or dark room you need to stay in till you finnish every single last lesson. Furtermore its no "betrayel" if you want to gather informations about other topics such as anatomy. Im just gonna say it as it is, as the same name already suggets "draw-a-box" is no complete guide to every single field of the spectrum of art, that means that you will sooner or later will need to do other courses if you want to gather more skills. Draw-A-Box strengthens fundamentals, which will always be very very important. You learn how to construct primitive form and how to put them into perspective. Essentially things will always run down to these fundamentals, thats why it is so important. You will soon realize how anatomy becomes easier for you because you are able to abstract those organic shapes into simplier ones. Thats why i think that putting other courses in addition to draw-a-box might help you actually apply those things you learned in draw-a-box to other topics.

Now to still warn you and not let you dive into some rampage of doing new courses every week. Just dont overgrind. Remember the 50/50 rule uncomfortable talked about. Always keep yourself a piece of fun that you can come back too. Im not saying that courses are always super dull and stuff but i think you already know what i mean. If you expect too much of you, you will just burn-out and stop drawing. We all dont want that.

With what Piggybird said, remember, it was stated at first that you shouldn't study in a bubble.

Sticking to one resource isn't useful in any skill you want to achieve. For example, if you want to learn Spanish and you just buy a single Spanish learning book, you will not go very far. Some intricate details that is forgotten to mention or isn't apparent to the author means that you also miss that point. You have to gather information from multiple sources, like how you would also watch Michael Hampton or Proko for example; you want to hear how native speakers speak, like how you would watch how master artist draw; you want to speak and make an output, like how you would doodle and experiment outside the course...

Of course, if you are casually learning it, sticking to one resource is fine. There's nothing wrong with being casual, some people just want to enhance their world views, some people just trying to draw without mastery level, or you may be just a bored guy looking to spend time. I feel like you are not trying to do this casually, as I just wanted to tell this to whoever may need it :P

You already stated that you will abide by the rules of the course and don't mix-and-match with other courses, so that is perfect.

Have fun in your journey!

Well It can go either way depending on your situation... and your current drawing abilities.

If you don't have enough free time to do both it may became counter-productive as you may be too tired to focus on Draw a Box's lessons.

Even if you have free time but you drawing abilities are the one of a beginner you may need time to make good drawing / get a result that satisfies you when submitting your homework.

In thoses cases, I would wait to have completed draw a box before starting another lessons.

If you're not in this situation, you may be able to be able to do both at the same.

I hope that this piece of advice had been able to help you.