In a letter dated March 24, 1937, Charles Moore, Chairman of the Commission of Fine Arts (CFA) relayed comments made by Paul Manship, the sculptor member of the CFA, to Nellie Tayloe Ross, Director of the US Mint, regarding the Battle of Gettysburg coin models that it had reviewed. The models were prepared by Frank Vittor, the designer/sculptor of the coin. (Note: The CFA had examined photographs of the models vs. the models themselves.)
Per Moore, Manship had commented, "the faces and expressions of the eyes of the two soldiers are nearly identical but [he] was aware that the artist may have done this intentionally."
This may truly have been Vittor's intent, or it may have had a more pedestrian cause/explanation - Vittor used only one model for the coin. James Power Sankey of Pittsburgh, a Civil War veteran (Union side), sat for Vittor as he prepared the coin's obverse design of conjoined portraits of a soldier from each side of the battle. One model, two portraits, similarities were inevitable.
Sankey died in November 1938 at the age of 91. This would have made him 89 or 90 at the time he sat for Vittor. Sankey joined the Union Army in 1863 at the tender age of 16; he was fortunate to not be injured during his service time. Sankey was also fortunate to be mobile enough to attend the 1938 Gettysburg Blue and Gray Reunion, a driver behind the coin's issue. (I wonder how many times Sankey told the story of his modelling session(s) to his fellow veterans?)
1936 Battle of Gettysburg 75th Anniversary Half Dollar

For more of my topics on commemorative coins and medals, including more on the history and designs of the Gettysburg half dollar, see:
Commems Collection.