`git log --name-only` show coauthors

You can, in that it is a trailer:

git log --format="%h %s %an Co-author:%(trailers:key=Co-authored-by)"

While it is true Git knows nothing about it, the trailer scheme allows to add any key=value you want to the commit message.

Since Git 2.32 (Q2 2021), you can make a commit with any trailer you want.

git commit --trailer "Signed-off-by:C O Mitter <committer@example.com>" \
           --trailer "Helped-by:C O Mitter <committer@example.com>"

how can I make git show the coauthors

You can't. Git commits don't have coauthors. There is a GitHub coauthor feature, and other hosting milieus may support that sort of thing too; but Git itself knows nothing of it.