This prefix is overused.
Certainly in co-conspirator, where it is unnecessary, as co- and con- both import the notion of joint action.
That doesn’t stop US legislators from perpetuating the redundancy, however: see, for example, Rule 801 of the Federal Rules of Evidence (where – strike two! – it is spelt without a hyphen).
Co- also appears a lot in modern job titles: She is co-head of the financial services practice group or The firm appointed two new co-managing partners.
This isn’t wrong, but somehow it’s not pretty – especially where the shared position has two elements like vice-president or managing partner.
Joint would be a more elegant substitute?