I say Z for Josiah and S for Josephine usually.I think it's based on which syllable I emphasize, which depends on speed...fast jo-SIE-ah jo-SEF-eenslow JOZ-ie-ah JOZ-uh-feen But I'm not sure why I usually say Josiah slower and Josephine faster.
I say Josiah slower than Josephine too. That "aia" sound is easy to say long, but all the vowels in Josephine are broken up by consonants I guess. I might say Josiah with a z (but usually with an s). I don't emphasize different syllables though, just the "SI" in Josiah and the "JO" in Josephine.