Cloud's persona was altered due to having his memories scrambled by Mako poisoning so he took on Zack's persona (at least how he perceived it)

While Sora has a second "heart", he is completely unaffected by the second presence within him until KH2 when he obtains the Anti-Form (which is more an expression of Vanitas than Roxas) and Final Form (an expression of Roxas). But this is a change in ability, not persona.