Turlough is both of them surnames[]
We could call him "Malkon Turlough" b\c they're brothers, right? So, they would have the same last name, unless Vislor invented a last name for himself when he came to Earth, or he was adopted by the Turlough family and took that as his name.
(Or is "Turlough" his middle name? Maybe we do not know Tegan s' surname at all, and "Jovanka" is actually "Tegan" s' middle name. Think of like Sarah is also known as Sarah Jane, only here we know her last name has always been Smith, while the Fifth Doctor era decided to drop the family identifier for the companions.)