Can Atmel Single Wire Interface (SWI) Support more than one device on the bus?
Section 1.2 page 5 of the datasheet


  Using the Single-Wire Interface, multiple ATSHA204 devices can share the same bus, which saves processor GPIO usage in systems with multiple Clients such as different color ink tanks or multiple spare parts. See Section 5.4, “Sharing the Interface” and Section 8.6.11, “Pause Command” for more details on the way in which this is implemented.


Section 5.4 page 21


  5.4 Sharing the Interface
  Multiple CryptoAuthentication devices may share the same interface, as follows:
  1.System issues a Wake token (Section 8.4, “Wake Sequence”) to wake-up all devices.
  2.The system issues the Pause command to put all but one of the devices into idle mode. Only the remaining device then sees any commands the system sends. When the system has completed talking to the one active device, it sends an idle flag, which the idle devices ignore but puts the single remaining active device into the idle mode. See Section 8.6.11, “Pause Command” for more details.


So SWI can support multiple ATSHA204's on a bus