Converted Symbol can't be Pasted into Keyframe: Motion Tweening

I have two files one is called tween_start.fla and the other is called tween_end.fla I am trying to create a motion tween where the animation begins at tween_start.fla and ends at tween_end.fla Each file consists of 7 shapes (it's a tangram if you're curious). At frame 10 of the tween_start.fla timeline I right-click on each layer and do Insert Blank Keyframe. Then I go to tween_end.fla click on a shape and do Modify>Convert to Symbol. Then I go back to the timeline for tween_start.fla, right-click on frame 10 and do Paste Frames or Insert Keyframe. If I do Paste Frames with all of the symbols I've created in tween_end.fla then the shapes end up stacked on top of each other. If I do Insert Keyframe it doesn't insert the symbol I've just created, instead it just keeps the same shapes that were at the first frame of tweet_start.fla. So how do I insert the symbols from tween_end.fla into the tenth frame of tweet_start.fla and then execute a motion tween between the frames?

At another forum, somebody thought that I was pasting multiple frames from tween_end.fla into a single blank keyframe in tween_start.fla but I'm not doing that. I have created the same number of blank keyframes on multiple layers. The layers are the same in both tween_start.fla and tween_end.fla.

How does the timeline know that what you are pasting into the keyframe is the new symbol that you have just created? There is no command that says Copy Symbol and when you paste into the keyframes you can only choose Insert Frame, Insert Keyframe, or Paste Frames so how do you get it to paste the symbol you just made.

This was just a non-standard way of creating a tween that had worked once accidentally but never worked again. I went back to the tutorials and learnt the standard way of doing it and then it worked.