Both Tom Felton and Natalia Tena appeared in the Harry Potter movies. Felton as Draco Malfoy, Tena as Nymphadora Tonks. Their characters in these films were cousins.
Lana (Natalia Tena) and "Eric" (Jóhannes Haukur Jóhannssen) both starred in Game of Thrones, as Osha and Lem Lemoncloak, respectively. They had no episodes together though.