disable multitrack recording for tasproj movies, the concept doesn't make sense, tastudio easily gives you multiplayer functionality by design. Delete some code that attempted to handle this scenario that likely didn't really work

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

@@ -318,6 +318,10 @@ namespace BizHawk.Client.Common
 				{
 					Output("Multi-track can not be used in Full Movie Loadstates mode");
 				}
+				else if (Movie is ITasMovie)
+				{
+					Output("Multi-track can not be used with tasproj movies");
+				}
 				else
 				{
 					MultiTrack.IsActive ^= true;

src/BizHawk.Client.Common/movie/tasproj/TasMovie.cs

@@ -182,9 +182,8 @@ namespace BizHawk.Client.Common
 
 			var newLog = new List<string>();
 			int? timelineBranchFrame = null;
+
 			// We are in record mode so replace the movie log with the one from the savestate
-			if (!Global.MovieSession.MultiTrack.IsActive)
-			{
 			if (Global.Config.Movies.EnableBackupMovies && MakeBackup && Log.Count != 0)
 			{
 				SaveBackup();
@@ -231,43 +230,6 @@ namespace BizHawk.Client.Common
 
 			Log.Clear();
 			Log.AddRange(newLog);
-			}
-			else // Multitrack mode
-			{
-				// TODO: consider TimelineBranchFrame here, my thinking is that there's never a scenario to invalidate state/lag data during multitrack
-				var i = 0;
-				while (true)
-				{
-					var line = reader.ReadLine();
-					if (line == null)
-					{
-						break;
-					}
-
-					if (line.Contains("Frame "))
-					{
-						var strs = line.Split(' ');
-						try
-						{
-							stateFrame = int.Parse(strs[1]);
-						}
-						catch
-						{
-							errorMessage = "Savestate Frame number failed to parse";
-							return false;
-						}
-					}
-					else if (line.StartsWith("LogKey:"))
-					{
-						LogKey = line.Replace("LogKey:", "");
-					}
-					else if (line.StartsWith("|"))
-					{
-						SetFrame(i, line);
-						i++;
-					}
-				}
-			}
 
 			if (!stateFrame.HasValue)
 			{