Hello Jodene, and happy hunting on your geneology. Bryan Howerton has the best information for your search, but I can give you a place to look (you will notice that this page notes Bryan as a source).

http://www.couchgenweb.com/civilwar/23rdcoh.html

These are probably your two soldiers (with misspelling); both of the 23rd Ark. (The William A. Thompson of the 2nd Ark Mtd Rifles was killed at Murphreesboro (Dec. 62/ Jan 63) which would exclude him).

THOMASSON, WILLIAM A Pvt
Enl 6Mar1862 at Arkadelphia, AR by A.A.Pennington. Wounded and missing after the battle at Corinth, MS 50ct1862. Captured 9Ju11863 and paroled l0Ju11863 at Port Hudson, LA.

WILLIAMS, HANSFORD Pvt
Enl 6Mar1862 at Arkadelphia, AR by A.A.Pennington. Present through 30Apr1863.

A logical surmise would be that someone wrote that Hansford Williams was killed by a shot to the head after "the surrender" and that someone else, at a later date, interpreted that as meaning after the surrender of the Confederate army in 1865. This would explain William A. Thompson witnessing Hansford death. What do you think?