don't use Global.Config in bk2movies, this solution is a bit of shenanigans but it's the least shenanigans I could come up with

src/BizHawk.Client.Common/movie/MovieSession.cs

@@ -25,12 +25,14 @@ namespace BizHawk.Client.Common
 
 		public MovieSession(
 			IMovieConfig settings,
+			string backDirectory,
 			Action<string> messageCallback,
 			Action<string> popupCallback,
 			Action pauseCallback,
 			Action modeChangedCallback)
 		{
 			Settings = settings;
+			BackupDirectory = backDirectory;
 			_messageCallback = messageCallback;
 			_popupCallback = popupCallback;
 			_pauseCallback = pauseCallback
@@ -41,6 +43,8 @@ namespace BizHawk.Client.Common
 
 		public IMovieConfig Settings { get; }
 
+		public string BackupDirectory { get; set; }
+
 		public IMovie Movie { get; private set; }
 		public bool ReadOnly { get; set; } = true;
 		public bool NewMovieQueued => _queuedMovie != null;

src/BizHawk.Client.Common/movie/bk2/Bk2Movie.IO.cs

@@ -20,10 +20,9 @@ namespace BizHawk.Client.Common
 				return;
 			}
 
-			var backupDir = Global.Config.PathEntries.MovieBackupsAbsolutePath();
 			var backupName = Filename;
 			backupName = backupName.Insert(Filename.LastIndexOf("."), $".{DateTime.Now:yyyy-MM-dd HH.mm.ss}");
-			backupName = Path.Combine(backupDir, Path.GetFileName(backupName));
+			backupName = Path.Combine(Session.BackupDirectory, Path.GetFileName(backupName));
 
 			Write(backupName, isBackup: true);
 		}

src/BizHawk.Client.Common/movie/interfaces/IMovieSession.cs

@@ -66,5 +66,7 @@ namespace BizHawk.Client.Common
 		void ConvertToTasProj();
 
 		IMovie Get(string path);
+
+		string BackupDirectory { get; set; }
 	}
 }

src/BizHawk.Client.EmuHawk/MainForm.cs

@@ -282,6 +282,7 @@ namespace BizHawk.Client.EmuHawk
 			Global.FirmwareManager = new FirmwareManager();
 			MovieSession = new MovieSession(
 				Config.Movies,
+				Config.PathEntries.MovieBackupsAbsolutePath(),
 				AddOnScreenMessage,
 				ShowMessageCoreComm,
 				PauseEmulator,
@@ -881,10 +882,10 @@ namespace BizHawk.Client.EmuHawk
 		private ToolManager Tools => GlobalWin.Tools;
 		private DisplayManager DisplayManager => GlobalWin.DisplayManager;
 
-		private IMovieSession MovieSession
+		public IMovieSession MovieSession
 		{
 			get => Global. MovieSession;
-			set => Global.MovieSession = value;
+			private set => Global.MovieSession = value;
 		}
 
 		private GameInfo Game

src/BizHawk.Client.EmuHawk/config/PathConfig.cs

@@ -254,6 +254,8 @@ namespace BizHawk.Client.EmuHawk
 				var pathEntry = _config.PathEntries.First(p => p.System == t.Parent.Name && p.Type == t.Name);
 				pathEntry.Path = t.Text;
 			}
+
+			_mainForm.MovieSession.BackupDirectory = _config.PathEntries.MovieBackupsAbsolutePath();
 		}
 
 		private void DoRomToggle()