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ok so suppose u have 3(or more) gifs to tween each with same/diff no. of frames:
1)Make all the gifs(if you want you can color them and save them and then tween or just get them resized and everything but dont color and save till all tweened).
2)Make new document.(size your choice coz ur gifs might be or not be of same sizes and frames)
3)Get the 1st gif of yours on this new document by paste frame(paste over selection).
For ex: 1st gif has 4 frames , and u have pasted it on the new document.
4)Now select the last frame(4th) and paste gif 2(paste after selection).
For ex: 2nd gif has 8 frames and it will start from frame 5 and end in frame 12.
5)Now select the last frame(12th) and paste gif 3(paste after selection).
For ex: this gif will start from the frame that will be after 12 and go till end.
6)Now all gifs are overlapped on each other but have separate frames. If you still want more overlapping then position some more.
7)Tween: select frame 5(first frame of gif 2) , press tween button n do this:


Add frames no. is ur choice(i do 3 or 5).
No. of frames has also increased (3 news frames got added by tween).
You'll notice that frames 5 , 6 , 7 have become translucent and if you scroll your layer tab you'll se two layers visible at the time(this doesnt happens for gifs wen not tween)

also opacity of these layers will not be 100 but reduced to two diff. values:

Now select the 3 translucent frames(ctrl+ left clicks) and increase the opacity of the below layer to 100%:
Run your gif and see the overlapping.
Now select the first frame of gif 3(frame 13th) and repeat the tween settings till the step above^(tween/select frames/increase opacity).


Now keep adding gifs if want more.
Now to create a loop, meaning tweening the last gif with the first gif we do this:
Select the last frame of your very last gif. and do this(note the tween between is changed):

You'll get 3 translucent frames and two decreased opacity layers(the topmost layer in ur layer tab and the bottommost).
Now we increase the opacity to 100 of the bottommost layer(layer1):

All done!!!!!!!!!!!!1