I think it could work, -wen and -gwyn- do sound similar, but I find the wyn and wen sounds different enough [name_f][/name_f]- especially with the strong G and the different ending.
[name_f][/name_f]It might feel different if it was [name_m]Benton[/name_m], [name_f]Arwen[/name_f] and [name_f]Gwen[/name_f], or [name_m]Benton[/name_m], [name_f]Bronwen[/name_f], and [name_f]Arwen[/name_f], but I find [name_f]Gwynna[/name_f] distinct enough myself