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.