For years, progress in the emulation scene stagnated when in came to Game Boy Advance to GameCube connectivity; a feature used in some games. This is because the task is no easy one as it requires thorough understanding of the two systems. However, with its 0.9 release, popular GBA emulator mGBA emulator brings support for connecting to Dolphin as well as some fixes on Dolphin's end of the GBA <-> GCN protocol to improve stability when connecting with any supported GBA emulator (not just mGBA).
The official Dolphin Emulator YouTube channel released a new video today testing the feature on several games, concluding that connectivity is better than ever:
However, a new article on the Dolphin Emulator website notes that there are certain issues with some games like The Legend of Zelda: The Wind Waker and Final Fantasy Crystal Chronicles. These issues can be attributed to the latency with the current protocol. That said, the Dolphin Emulator team says that they have a plan to address those issues by integrating the emulators more closely and include mGBA features in the process like save states and netplay.
You can check the Dolphin Emulator article in the source link below for more details about this feature and for a compatibility list.