Circuit for Atmel (now Microchip) Single Wire Interface (SWI) using actual TTL UART?
No, standard TXD and RXD cannot be connected together as TXD is always an output from Raspberry Pi or other TTL UART. It would push data line high while the Atmel chip needs to pull the data line low to communicate back to RXD pin.
If it is possible to make the TXD output as open-drain then it is possible to connect TXD and RXD together. Otherwise you need an open-drain buffer to handle that.