Why does ATA Secure Erase occur concurrently rather than in parallel with PATA drives?

When issuing the ATA secure erase command via hdparm against multiple SATA (non-SSD) drives it occurs in parallel. However when the same command is issued against PATA drives, it occurs consecutively. For example the second PATA drive does not commence its process until the first process has completed. Is the ATA Secure Erase command limited by a single PATA channel ? If yes, why would it be since its an internal routine of the drive controller? Can it be overcome with independent IDE channels? Note in issuing the dd command to wipe the drive, it occurs in parallel. PATA drives have historically been is use in aging and legacy devices that are now being decommissioned. The requirement is to securely wipe the drives as they contained sensitive data such as personally identifiable information.