Personally, I think it's important not to pick matchy names for twins, as it's all too easy for people to forget that they have individual personalities, and simply think of them as 'the twins.' If they have overly cutesy matching names, it makes this even more likely.
So maybe names that are similar in style, but not too similar, such as:
Emily and Catherine
William and Thomas
Ruby and Grace
Helen and Anna
Harry and Charlie (or Henry and Charles, but not Harry and Charles/Henry and Charlie)
Benjamin and Nicholas