Programming several AVRs with one ISP: which line to break?
break MOSI because that's an output back to the host
and break SCK so the other MCU can't see the programming commands.

edit - this still won't work as reset will be pulsed on and off during the programming. ther other data lines also need to be broken, or reset for the other MCU(s) held low.

I guess that's why most designs use a separate programming header for each MCU.