Halo 2 Online Co-op crashing

So my girlfriend doesn’t really play games, but she’s been happy to try Halo with me. We beat Halo CE without any issues, but it was different when it came to Halo 2. If we tried playing Halo 2 co-op on different consoles, one of us would crash whenever we tried to start a mission. As a result, we’re forced to play splitscreen if we want to play the game together.