The shadows were obviously Marty and Doc. Telltale ended Back to the Future on a cliffhanger and said there would be a season two but didn't. It was also their most successful title until TWD. Only obvious that there would be a crossover as they crossed dimensions by time traveling too much. Besides, why else would that car be shown near Clementine in the ending? They made it look different so the shadows wouldn't be so easy to figure out.
If not them, then Lee and Kenny, I have a feeling if you cut off Lee's arm and left him to die, he survives as he was just unconscious. If you left his arm then Lee wakes up and chops it off with the cleaver he had or the broken glass from the vase. Even if you handcuffed him, his bad arm was not handcuffed. He will use it to free his other arm. If he has one arm only and cuffed then Kenny will come and save the day again by freeing him.
If you had Clementine shoot him she probably missed and only grazed his head just like when B.J. shot Chase and he survived by a graze in the Walking Dead webisode mini-series "Cold Storage". Makes perfect sense why Clementine would look so shocked at the epilogue.
Another likely possibility is Campman's son and Guybrush Threepwood. Telltale would make him a cameo appearance with Campman's son and he will shoot Guybrush at the very beginning of season two and try to kidnap/kill/eat/molest Clementine. Just like how Campman's son betrayed his parents, he would betray his survivor.
If none of those are right, only then we can say that the preposterous idea of Omid and Christa are the two shadows.