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You could use tee and process substitution to output to multiple disks e.g.

dd if=masi.img | tee >(dd of=/dev/disk2) >(dd of=/dev/disk3) | dd of=/dev/disk4

Alternatively, there's dcfldd which is some enhanced version of gnu dd and it can output to multiple files or disks at the same time:

dcfldd if=masi.img of=/dev/disk2 of=/dev/disk3 of=/dev/disk4