How to list co-first authors and co-second authors?
Like @FuzzyLeapFrog writes in a comment, you could use any symbol in your CV as long as you explain it, e.g. in a footnote. There is no clear standard as far as I know. Personally I'd use a parenthetical remark to point out that I'm a co-first author, but you say there's no space. In that case, I'd just use whatever symbols are commonly used in papers to denote affiliations or co-authorship in your field. Often this means typographical symbols called daggers that are used to denote footnotes after the asterisk has been used. The order tends to be


Asterisk: *
Dagger (or obelisk): †
Double dagger (or diesis): ‡


In case you really want to go overboard with it, Wikipedia has a few suggestions for a fourth symbol, but there is no real convention for what the fourth symbol should be. Also note that the dagger symbol is sometimes used to indicate that the author passed away, so you may need to take some care with that in some contexts.