Whereas the 20thís flag had 34 stars, the 34th being for Kansas, the 26thís flag had 35 stars, the 35th being for West Virginia, which split off from Virginia over the issue of secession. Another difference: the 26thís had its name embroidered on one of the red stripes.
20th & 26th USCT flag images can be found on the web site of the NYS Military Museum & Veterans Research Center at:
http://www.dmna.state.ny.us/historic/btlflags/infantry/20thInfUSCTCamp.htm
http://www.dmna.state.ny.us/historic/btlflags/infantry/26thUSCTNational2005.0076.htm
http://www.dmna.state.ny.us/historic/btlflags/infantry/26thInfReg.htm