I think they're both lovely but I'm not sure they 'go' together. Which might be exactly what you want with twins but, imo, they're just a bit too different.
I would Arabella with something like:
Charlotte, Cassandra, Delilah, Elizabeth, Emmeline, Evangeline, Francesca, Felicity, Georgina, Hermione, Imogen, Joanna, Juliette, Katherine, Lucinda, Madeleine, Miranda, Matilda, Natasha, Natalie, Olivia, Philippa, Rebecca, Rhiannon, Romilly, Sophia, Tamara, Verity, Victoria, Vanessa, Xanthe, Zara
And Everleigh with something like:
Amberley, Addison, Avery, Blair, Bailey, Casey, Cassidy, Dakota, Devon, Fallon, Genesis, Hadley, Halle, Hayley, India, Jasmine, Kayleigh, Kennedy, Leonie, Layla, Lola, Macey, Miley, Madison, Mackenzie, Mackenna, Nyah, Paige, Piper, Peyton, Quinn, Riley, Reese, Sutton, Saffron, Savannah, Sienna, Sierra, Sabrina, Taylor, Zillah