Can avrdude address individual usbtiny programmers?
Multiple USBasp or USBtinyISP devices can be identified by their connection information. From the AVRDUDE manual, appendix B, "Troubleshooting":


  
  Problem: Multiple USBasp or USBtinyISP programmers connected simultaneously are not found.
  
  Solution: The USBtinyISP code supports distinguishing multiple programmers based on their bus:device connection tuple that describes their place in the USB hierarchy on a specific host. This tuple can be added to the -P usb option, similar to adding a serial number on other USB-based programmers.
  
  The actual naming convention for the bus and device names is operating-system dependant; AVRDUDE will print out what it found on the bus when running it with (at least) one -v option. By specifying a string that cannot match any existing device (for example, -P usb:xxx), the scan will list all possible candidate devices found on the bus.
  
  Examples:

avrdude -c usbtiny -p atmega8 -P usb:003:025 (Linux)
avrdude -c usbtiny -p atmega8 -P usb:/dev/usb:/dev/ugen1.3 (FreeBSD 8+)
avrdude -c usbtiny -p atmega8 \
  -P usb:bus-0:\\.\libusb0-0001--0x1781-0x0c9f (Windows)