2 Pwr Supplies in 1 box?

[Castellen]

Should work fine, so long as the ground connections are common between power supplies.
Which they will be, the IDE/SCSI cables contain a number of ground connections which will do this for you automatically.

Using two power supplies in parallel will not cause them to blow up, the only thing that's going to happen is that if the output voltages are not equally ballanced between the two supplies, one supply is going to work harder than the other one.

[Castellen]

Exactly right.
Using a 2nd power supply inside the computer case to power hard drives, CDrom, etc is no different to external SCSI devices and the like having their own seperate power supplies.

I can't say it's the tidiest way to do something, but there's no reason why it wouldn't work.