Bonus points if those instances are Mastodon and/or kbin.
Fediverse
This magazine is dedicated to discussions on the federated social networking ecosystem, which includes decentralized and open-source social media platforms. Whether you are a user, developer, or simply interested in the concept of decentralized social media, this is the place for you. Here you can share your knowledge, ask questions, and engage in discussions on topics such as the benefits and challenges of decentralized social media, new and existing federated platforms, and more. From the latest developments and trends to ethical considerations and the future of federated social media, this category covers a wide range of topics related to the Fediverse.
What do you want to hear?
Both instances are single-local-user, both are able to find remote accounts on the fediverse including my own, but despite being on the same Docker network, they are not able to find each other's accounts.
Are you using a VPS or self-hosting?
Self-hosting, just for development purposes.