These two names don't start with the same letter but have the same kind of sound and are yet different. Ava and Eve. They're both versions of Chava (the original form of Eve, Eva would be too similar to Ava though) but these two sound okay. Plus you can use Evie as a nickname for one of them! They are both pretty, relatively uncommon (Ava is for sure and Eve is well as far as I know, at least nowadays, but it's a classic name) and palindromic too! I know if I ever had twin girls I would consider these two names. I think EveAlexandra and AvaElizabeth are beautiful names (plus the initials are a reversal of each other, another good commonality for twins names to have!) :)
I'd have to agree to leave Allegra off the list, it IS a type of medication. I personally don't care for names like Tammy (I don't think it's ever a good idea to use an abbriviation as a first name), Kaleigh and Kaitlyn, but it's all a matter of personal choice. Happy decision making!