Homebrew Redundant PC Power Supplies
You do not want to try what you are proposing. Differences in the two power supplies are likely to lead to big ripple or oscillations. And even in the best of situations the two common PC type supplies are not likely to share the current load equally. 

Adding the low drop diodes may seem like a good idea at first to isolate the supply outputs from back feeding but it does not prevent the problems mentioned above and could in fact make the problem worse.

Learn from how this is done in the server world where redundant power supplies are common to support maximal up time and hot swap out if a supply fails. Power supplies of this type are designed to work when connected up in parallel. There is a special signal wire that is common across all the supplies that allows management of the load current sharing between the supplies. Load current sharing works better when supplies have a minimal number of output voltages to manage. This is one reason that server power supplies have been reduced to providing just the 5V always on rail and a single 12V rail output. All other voltages needed by the server are produced either on a power distribution board that the redundant supplies connect to or on the server motherboard. (Sometimes a combination of both).

My take on this is that if you purchase a good quality PC power supply, keep it cooled with a good quality fan and regularly blow out all the dust that tends to accumulate over time that the reliability of the power supplies is likely to be better than the hard disk drives. If any thing you may want to consider using a NAS (network attached storage) packaging concept and configure your drives in a simple redundant RAID configuration (i.e. simple whole drive mirroring). Get an enclosure that lets you have your SATA drives in trays that can be easily swapped in and out.