...
I edit the flash-all.bat and pull out the lines, 1 at a time in a CMD windows with ADB and the firmware in focus (CD'd to that directory or path), boot, radio, then system all taken directly from the batch file with rebooting to bootloader from the device after each line completes. I watch the 3rd line as thats the one that wipes, then reboots, and immediately after the first slot is completed, it reboots to OS (but we dont want that to happen yet), so I am watching and waiting for that step. As soon as I see the vendors flash, I get ready to catch the process and about to hold volume down+power the very second it reboots. As soon as it reboots, I am holding power+volume down so it catches the reboot and enters bootloader again. At this point the first firmware has completely flashed slot-a. Then (back in bootloader), I switch to slot B through ADB, redo the entire thing ...
Possibly, there is an quicker way to do this using the "--slot all" switch and the "--skip-reboot" switch
You could replace the original flash-all bat from
fastboot flash bootloader bootloader-taimen-tmz20r.img
fastboot reboot-bootloader
fastboot flash radio radio-taimen-g8998-00008-1902121845.img
fastboot reboot-bootloader
fastboot -w update image-taimen-pq3a.190801.002.zip
fastboot reboot-bootloader
fastboot flash radio radio-taimen-g8998-00008-1902121845.img
fastboot reboot-bootloader
fastboot -w update image-taimen-pq3a.190801.002.zip
to
fastboot --slot all flash bootloader bootloader-taimen-tmz20r.img
fastboot reboot-bootloader
fastboot --slot all flash radio radio-taimen-g8998-00008-1902121845.img
fastboot reboot-bootloader
fastboot -w --slot all --skip-reboot update image-taimen-pq3a.190801.002.zip
fastboot reboot-bootloader
fastboot --slot all flash radio radio-taimen-g8998-00008-1902121845.img
fastboot reboot-bootloader
fastboot -w --slot all --skip-reboot update image-taimen-pq3a.190801.002.zip
(I have not tested it, but the process you describe seems quite lengthy)
Also, if someone is going from stock to havoc (or the other way around), he could possibly remove the "-w" switch and perform a dirty flash as data partitions seem compatible.
Theoretically, one could have a dual boot setup, and could switch from havoc to stock without flashing anything, just by activating the other slot ...