If the girls aren’t changing the way they speak when they’re undercover then he really needs to have his eyes checked
It's actually done reasonably well in the LNs. The more time they spend around each other, the more obvious it becomes who they really are. First-arc girl stays fooled for about five seconds once he helps take on her situation. Second-arc girl recognizes him, too, but is in some heavy denial about it (even her subconscious is like "take a hint girlie").