Can I charge multiple identical devices in parallel from on transformer
No, a single power supply designed for 12V 2A will not supply 12V 8A, and when asked to do so will likely shutdown or fail in some other way; low voltage, overheat, blow internal fuse.

The time this will happen is when it is most inconvenient; several minutes after some of the drives have spun up and have begun to write data.  Your data may not survive the experience.  That includes data already safely written.

You need a 12V 8A power supply to do this.

You also need to worry about the effects of the disk on the power supply, affecting the next disk.