If two people draw a picture, who owns the copyright?

If two people collaborate and produce a single picture, then as others have noted, they are “joint authors” and they jointly own the associated copyright. Among other things, that means a single author can unilaterally license the work to anyone and on any terms the single author sees fit. (There is, however, a requirement to share licensing revenue, if any, with the other coauthor(s).)

Copyright in Vietnam 

 But you might be asking if two people independently each draw two identical or near-identical pictures. In that case, they each own the copyright in their respective work, even if the works are damn near identical.

This is true even without regard to timing. If you produce something (whether it’s a picture, a poem, a song, etc.) and fifty years later I independently produce the identical work, then (a) I’m not infringing your copyright, and in fact (b) I own the copyright to my work, even though it happens to be identical to your work.

Of course, this is kind of an academic statement. If the first work is popular, it’s damn near impossible to prove the second work was produced independently. If the first work isn’t popular, then in practice it’s unlikely there will ever be a real dispute.