Reproduced on Windows with mGBA 0.9.2 and mGBA 0.10-7138-59cb5c189. How to reproduce the bug:

• Boot up Crash Bandicoot 2: N-Tranced (USA). This bug requires a specific game unlockable that isn't commonly found on save files uploaded online, so I will be uploading mine as the attachment in this issue (rename to .sav). Load the save file.
• Start a multiplayer window and load the same ROM, then go to the main menu and select LINK GAME -> ATLASPHERE on both windows.
• Both windows should be in a character selection screen now. On one of the windows select either of these characters: "N. Trance", "N. Tropy" or "Megamix". Go to the player 1 window and press start.
• Pause the game on the player 1 window. What happens after depends on if any of the two characters are being used by player 1 or player 2, and which window is used to open Sprite Viewer. For example:
• • Selecting "N. Trance" as player 1 and opening Sprite Viewer on any of the two windows will crash the emulator.
• • Selecting "N. Trance" as player 2 and opening Sprite Viewer on any of the two windows will open Sprite Viewer normally. No issue here.
• • Selecting "N. Tropy" as player 2 and opening Sprite Viewer on the player 2 window will crash the emulator. Opening Sprite Viewer on the player 1 window will open Sprite Viewer normally.
• • Selecting "N. Tropy" as player 1 and opening Sprite Viewer will make it open on the player 1 window and crash the emulator on the player 2 window.
• • Selecting "Megamix" as player 2 and opening Sprite Viewer on the player 2 window will crash the emulator. Opening Sprite Viewer on the player 1 window will open Sprite Viewer normally.
• • Selecting "Megamix" as player 1 and opening Sprite Viewer on the player 1 window will crash the emulator. Opening Sprite Viewer on the player 2 window will open Sprite Viewer normally.