Traditional social media platforms, once envisioned as digital town squares, now face growing criticism over corporate control, content moderation, and privacy concerns. Events such as Twitter's acquisition (now X) and major policy changes have pushed users toward alternative platforms like Mastodon and Threads. However, this diversification has led to user dispersion and fragmented discussions across the walled gardens of social media platforms. To address these issues, federation protocols like ActivityPub have been adopted, with Mastodon leading efforts to build decentralized yet interconnected networks. In March 2024, Threads joined this federation by introducing its Fediverse Sharing service, which enables interactions such as posts, replies, and likes between Threads and Mastodon users as if on a unified platform. Building on this development, we study the interactions between 20,000+ Threads users and 20,000+ Mastodon users over a ten-month period. Our work lays the foundation for research on cross-platform interactions and federation-driven platform integration.