I don't know enough about it but have you checked this manual ?
https://hddguru.com/download/documentation/AT … ATA-ATAPI-5.pdf

Hmmmm... It also says nothing about the exact way the DRV bit selects the drive.

Could it be that both drives always accept writes to all registers (except the command register), then use the DRV bit in their own device/head (or drive/head in ATA-1) register to determine if they're selected?

So that would mean that the value in the drive/head register for device #1 (the slave device) would always get destroyed when performing a SRST reset? Since the drive/head register would become overwritten when reselected?
The other registers (the signature) might be preserved, assuming that the master doesn't get the slave values overwritten by the host writing the master's command block registers?

So the only way to detect both master and slave CD-ROM drives using a SRST command is by ignoring the drive/head register and only checking the other registers by reading them and not writing them until both drives have their registers read first(except the drive/head register, which will always be overwritten when selecting the slave drive)?

Edit: Also, what happens if an ATAPI drive releases the bus due to an IRQ or starts raising an IRQ while the drive isn't selected? The other drive on the channel could have the interrupt reason register overwritten, together with any relevant data in the cylinder registers?

there's two persons on this forum which I'm sure could answer your questions or point you to the right info though neither is very active here.
electric_monk & sintech