@daddylonglegs as I already tried to explain to you: the Pixel 3a has two boot slots for seamless updates, i.e.
- before the update it starts e.g. from slot A
- after the update slot B is activated and the phone tries to start from slot B
if for whatever reason the phone cannot boot, slot B will be marked as 'bad' and the phone will automatically start from slot A again.
Flashing to both slots in parallel is not needed / recommended as there is also a ramdisk / system / ... connected to slot B and if those are not compatible to the boot image you have just flashed it will mark automatically the slot as 'invalid' and will fall back to the other one ...