Off the top of my head:
Superhuman,
Zero End, and
The Hellcat Lounge all have multiple alt characters. (Superhuman and Zero End also have phenomenal worldbuilding, great stories, and are two of my favorite AVNs around. I'd put Hellcat Lounge more in the "has potential" category at the moment.)
Being a DIK has Quinn and Lily.
Summer Heat has Eve and Helen.
New Coral City (play the old version rather than the remake because it has way more content) has Rain and Akissa. Come to think of it, Maud might qualify as well, but don't expect a love path with her!
Summer's Gone has Nia and
Where It All Began by the same dev has Katie and Leia.
Porn Empire has alt porn stars. There's also a special character, Alice, who is an alt model.
UPDATE: A couple more came to mind.
Our Red String has Ivy, Emma, Louise, and Jessica. I can't believe I didn't think of it right away!
Color of My Sound has Blanche, Mireille, and Safiya.
