I like it but have only seen it written Elowen. And I agree that two Ellies is a big risk. You obviously wouldn't call them both Ellie but their individual groups of friends might.
How about Ellaria and ...
Ariadne, Anoushka, Aven, Bianca, Briar, Bettina, Calliope, Calista, Celyn, Danica, Devora, Delphi, Ebony, Evadne, Emmanuela, Fenella, Fabia, Felicia, Giana, Guinevere, Genevieve, Honor, Harmony, Hebe, Isadora, Iona, Imelda, Jacinta, Jocasta, Juniper, Keziah, Kendra, Kiki, Lorelai, Leonie, Livia, Miranda, Marissa, Melinda, Nerilly, Neriah, Nadia, Octavia, Odette, Opal, Paloma, Pandora, Persephone, Rhiannon, Rosina, Rowena, Saffron, Saskia, Sephora, Tassita, Talitha, Tamara, Verity, Vivianne, Vanessa, Xanthe, Xanthippe, Zipporah, Zeryn, Zelda