Programming several AVR MCUs simultaneously? (with parallel)
If you use sufficiently conservative timings and ignore the MISO pins on all but one of the MCUs, it’s possible that you could program them in parallel. However, I don’t see how you could verify them in parallel unless you break out every MISO pin onto a separate pin and somehow check them all. 

Unless you have a need to program truly massive numbers of MCUs, the whole thing strikes me as more effort than it’s worth.