My two cents of comedy for this is as follows:
At the end of your second short film, perhaps Doc can exclaim somthing similar to "Oh heavens! I dropped by note book back at my old house!" Then, before Marty can say too much, a duplicate DeLorean shows up and an estatic Doc and Marty exit this second vehical saying "Woo-hoo, we got the book back in time!" then the newly arrived Doc and original Doc can both say "Drat, I've seen myself again, I hate it when that happens."
Hows that for an ending eh?
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