apparent facts:
1. Cousins who lived in the same area, both were born between 1830 - 1840.
2. both are grandsons of John (Revolution Patriot) & Nancy Everingham
3. Father - Samuel died in 1844
4. Father - Jeremiah died in 1880
5. Charles of Ononadaga, NY seems to be a little older than Charles of Manlius.

Charles "B" listed with his widow mother and siblings:
1850 CENSUS Manlius, Onondaga, NY (#592) ... Sarah Bradley 47,.. William Evingham 25, teamster,.. Geo Evingham 23, miller,.. Mary A. Evingham 25,.. Hannah Evingham 21,.. Angelina Evingham 19,.. Smith Evingham 17, boatman,.. John Evingham 12,.. Chet Evingham 10,..Orville T. Evingham 2. I believe that CHET listed here, is one of the Charles Everinghams.

From this 1850 info it appears that Charles "A" was from Onondaga, and Charles "B" was from Manlius, both in Onondaga County, New York.
1855 STATE CENSUS Manlius, Onondaga, NY (Page 405, family #287).. Sally Everingham 52, lived in this town 23 yrs,.. Charles Everingham 15, not married, son,.. Louisa Everingham 20, married, daughter-in-law,.. George Everingham 0, grandson,.. James Everingham 21, son, married, occ: Boatman. From this 1855 info Charles "B" was still in Manlius with various family. From 1850 and 1855 Census data, it appears that Charles "B" was born about 1840.

Charles "B" listed with his brother and widowed mother:
1860 CENSUS Manlius, Onondaga, NY (Page 80, family #567) ... John Evringham 21, b.NY, day laborer,... Sarah Evringham 57, b.NY,... Charles Evringham 18, b.NY, day laborer.

not sure which Charles this is, but he was born about 1841 which fits best with Charles "B":
1862 EVINGHAM, Charles, res. Fayetteville, NY, private, 122 NYV, Co. C., enlisted Syracuse, N.Y., mustered in Aug. 28, 1862, deserted. (name spelled Evingham per "The Fayetteville Gazette," Aug. 7, 1862.) (Evingham was 21 when he enlisted on Aug. 13, 1862 ... and was a private when he deserted on Sept. 30, 1863, Swinfen.) ... compare to the brother of Charles "B" - John Evingham, same unit - 122 NYV.

Charles "B" listed with his brother;
1870 CENSUS DeWitt, Onondaga, NY ... Charles Everingham 27, farm laborer, b.NY,.. Mary Everingham 26, b.NY,.. John Everingham 32, b.NY, farm laborer,.. Mary Everingham 24, b.NY,.. Arthur Everingham 3. This seems to indicate that Charles of Manlius was now in DeWitt, and was married to a "Mary" and was living with brother John and his family. In a State Census (below) on Onondaga County, he is back in Manlius and has 1 child
1875 NY State CENSUS, E.D.2, Manlius, Onondaga, NY (Page 50, Fam#137) Charles Everingham 33,.. Mary Everingham 32,.. Lilly May Everingham 1. This Charles was born about 1842 which is still more consistant with Charles "B" along with living in Manlius.

MAP of Onondaga County, New York.

1880 CENSUS Fayetteville, Onondaga, NY (Page 18A #1533) ... Charles Everingham 39,.. Mary 34, wife,.. Lillie 6,.. Harvey 3. Mystery here is... which Charles & Mary is this?... it apears to be Charles "B" because of the Fayetteville connection. (See 1862 info above & 1888 below)

1884 This is the only record I have found to prove a marriage of either Charles; This is Charles "A"
NE Marriage Record -- Charles Everingham 44, b.NY, s/o Jeremiah Everingham & Sophia French,... Mary A. Reel 36, b.MI, d/o James Shepherd & Elizabeth A. Brower,... m.5 Oct 1884 at Fairmont, Fillmore, Nebraska
Notice this Mary was born in Michigan... The other Mary in Census data was born in NY or Ireland or parents born in Ireland.

1888 -according to The Fayetteville Gazette, Charles Evringham of Onondaga County, NY who served during the Civil War was Living in Fayetteville in 1888. "This was most likely Charles "B". Fayetteville, DeWitt and Manlius are all neighboring villages in Onondaga County, New York. (see map above)