Ken,
Here is what the NY Military Museum roster has on Barrow:

35th Infantry
BARROW, DENNIS.—Age, 18 years. Enlisted, September 15,
1861, at Watertown, to serve unexpired term of two years; mustered
in as private, Co. E, September 18, 1861; mustered out
with company, June 5, 1863, at Elmira, N. Y.; subsequent service
in Eighteenth Cavalry.

18th Cavalry
BARROW, DENNIS.—Age, 21 years. Enlisted, August 15,
1863, at Watertown; mustered in as private, "unassigned,"
October 14, 1863, to serve three years; no further record; prior
service in Co. E, Thirty-fifth New York Infantry.

And reposting their record of Beneaux in the 35th for comparison:
BENEAUX , STANISLAS.—Age, 19 years. Enlisted, September
14,1861, at Watertown, to serve three years; mustered in as private,
unassigned, September 18, 1861; no further record.

No mention of a promotion to sergeant, or service in the cavalry as a lieutenant, or of service in the Maine cavalry.