Yep. It is possible for Kotlin and Java to live side by side inside the same app. That’s what I did with rider’s app for Karhoo Courier. For the purpose of learning, I only use it in one of the activity. As I said before, the best way to learn is by using what you want to learn with what you have to do.
If you can combine them, you can learn while achieving your objective of the day (for me, it is to let the riders view the consignment note details).