Is it ok to connect the output of buck regulator in parallel?
Directly connecting the outputs of multiple regulators, switched or linear, is inadvisable for the following reasons:
A marginal difference in output voltage would cause high currents to flow between the regulator output pins, potentially damaging one of the regulators.
The MCP16322 is rated for 2% precision, hence for a 5 Volt nominal output, one regulator could be at 4.9 Volts, the other at 5.1 Volts. The 0.2 Volt gap would cause current flow between outputs limited only by the rail impedance of the regulators.
Any delay in powering up or powering down of either regulator would cause a back-feed from the powered regulator to the non-powered one.
By design, the approach stated in the question will have one of the regulators operating while the other may not be - if one of the power sources is off at a given time. This is a failure mode with strong likelihood of device damage
Even if the two regulators were powered by a common source, there will be mismatches in power-up timing while the two oscillators are starting up. This is why sequencing of power supplies is required, and there are special-purpose parts for this sequencing.
There will be higher peak voltage / peak current demands on output stage capacitors of the regulators, due to additive effects of the (non-synchronized) ripple voltages of the two.
A buck controller that supports synchronization and sequencing would be required, instead of the selected device. If the design proposed in the question is used as-is, even if there is no immediate failure, component deterioration would reduce the expected longevity of the device due to repeated exposure to stresses not designed for.
The solution:
Instead of a diode-OR of the outputs of the two buck regulators, use diodes to merge the 12 Volt input sources. The design can then use a single buck regulator instead of multiple. The datasheet indicates that the regulator will not have any trouble using a 11.3 Volt input instead of 12 Volts, to produce a regulated 5 Volt output as desired.
This article about sequencing of multiple voltage rails might be useful reading, it discusses the sequencing and component degeneration issues.
It is generally not a good idea to parallel the output of two power supplies. Both power supplies are unlikely to be at the same exact output voltage. As a result one will tend to try to supply all of the load whilst the other one will tend to idle along at low load. Depending upon the filtering characteristics used in the feedback networks on the two power supplies it is possible that oscillation could also happen.
Now all that said there are power supplies designed that are specifically designed to be able to be paralleled. These often have a special sense line that connects between all the power supply outputs that is used to support a balanced current sharing between the supplies. Designs of this type are more expensive and do add additional components to the circuit board. Current sharing supplies also have to add additional levels of fault detection to ensure safe operation/shutdown in the event that the common current sharing scheme fails or some component in a particular power supply fails.
It is not uncommon to see this type of parallel usage power supply used in server computers where power delivery is added in modular manner to the server as additional CPUs, memory and I/O boards are added to the server. Many of these power supplies contain internal microcontrollers that run sophisticated fault detection algorithms to make them safe in failure modes.
To overcome the problems due to parallel connection (mentioned in earlier answers) you can make some circuit modifications as listed in this article at Electronic Design or this application note by Texas Instruments.
Of course it all depends from circumstances but this recommendation may help you to enlarge current capability of yours design.