Using another person to motivate I am learning  various topics on my own, and so is another person that I know. We were planning on studying together around four days a week, a half an hour at a time (perhaps to be increased if all goes well) to help motivate each other.

However, we're studying entirely different topics. Like, not even machine learning and complexity classes different. More like accounting and computer science different. What are some ways to motivate each other and improve each others learning even though we're not learning the same things and we don't have a ton of interest in each other's topic?

I suggested occasionally sharing a "big idea" the other learned, but I was wondering if there were some other things we could do.
I would suggest that you start each "session" with each one of you going over what was learnt in the previous session. Devote ~15 mins for it. 

This is not in spite of the fact that you "don't have a ton of interest in each other's topic", but because of it. This "review" of the last session gives, as I see it:


You are exposed to some other field. Who knows? you might end up finding interest in it. Even if you don't having someone listen to one's recount of one's learning provides a platform for a sanity check. The person telling their friend checks each part of what was learnt, as they talk.
It makes it easy to "get back in the game", and you continue learning after those 15 minutes, as though the previous session was just moments ago. This is especially useful, and is overall what makes this suggestion worthwhile. 


Plus, showing interest in what your friend is learning is nice, and no one1 ever got hurt from learning something new in an unrelated field.

To elaborate on what I wrote as 1. above:

The sanity check and telling about what you learnt are also useful, because ideally, you'd lower the stem of details. You'd want to keep it to examples and simple explanations. This is extremely good for your own learning2 because as you explain the material, after reducing the number of details and making it simple, you focus on the core of what you learnt. It might be difficult at first, but that's why you each have 15 minutes. Don't be afraid to extend it to 20 if need be. This is a very important part of my suggestion.

Over time you'll be able to extract the core of the material more quickly, and that's a useful skill, unrelated to learning in pairs.

I highly recommend this technique (you can modify it to your hearts content; I won't hold it against you :P). I employed this method with a friend and, although we were both studying CS (but different branches; I was studying machine learning and statistic approaches and he was studying networking), it was very useful (also self-learners ;)) to both learning the subjects and the aforementioned skill.




1That's probably incorrect, but it's highly unlikely that listening to what a friend learnt in accounting would do any damage.
2and for your friends, when they talk about what they learnt