You said you measured the 1.3 Amps across Vin and GND. This is NOT the way to measure the current capability of a power supply - it does give you the short circuit current, but that is usually not a useful value. If the existing power supply claims to be 12 volt and 900 mA, you should believe that current rating, and not attempt to draw more currrent.
To measure current, you must connect your meter in series with the circuit - you break the circuit to do this.
You should measure the actual current drawn by the 12 volt load. If it is less than 900 mA, then the difference is the current you have available to power your step-down converter.
As others have said, the 300 mA rating of the stepdown converter is the maximum it can supply, not what it will actually draw. Since the stepdown ratio of the converter is 5/12, we can expect the current drawn from the 12 volt supply to be a bit more than 5/12 of the load current - perhaps 35 mA. (As a rough approximation, we can assume that the power into the converter equals the power out, plus some losses in the converter.)
The total current drawn from the 12 volt supply will then be whatever you measure for the scale, plus the 35 mA or so for the step-down converter and 5 volt load.
Those regulators are not designed for such high-power applications. You can't connect them serially because they were not made to be poly-phase. If you try to connect them in this manner, the feedback loop in the converter will freak out and can cause very unexpected results which may damage whatever you are trying to connect it to.
Your required large input voltage range poses kind of a problem. I'm having a hard time finding a converter that can accept up to 400V.
Regardless, you are going to need a much more suitable solution. The converters you have are not fit for this in any way. You will want to be looking for buck switching mode power supplies
EDIT: Generally you would convert the 3-phase AC down using a transformer. Common DC voltages output by the transformers are around 24v, 12v, among others and this approach is much more easily usable by other switching mode power supplies.
Best Answer
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.