I agree with
Claudia -- the spelling of these names is a matter of personal preference.
With the particular example of
Joanne, though, I think it's probable that one could say that those who respell this as two separate names, with or without the hyphen, are emphasizing that they are reinterpreting the name as being a blend of
Jo and
Ann rather than just a French form of
Joanna. So I think the Jo-Anne, JoAnne, etc. spellings probably are even more likely to be thought of as feminine forms of
Joe or
Joseph than the original spelling
Joanne is.