paralleling switching regulators
It is possible to do, but it is not as simple as you think. 

First comes the switching frequency. If you have several regulators interacting, beat frequencies and resonances can occur. As you mention some modules/regulators have synchronization inputs and/or outputs that would fix this issue. 

Then comes load sharing. Whatever module has the highest output voltage will take the brunt of the load while the other regulators attempt to reduce their output. A small series resistor on each might help equalize the loads somewhat, but this will waste power and reduce load regulation.

And last, but most importantly, comes regulation feedback loop stability. The interaction of multiple similar feedback loops fighting with each other, makes an overall unstable and chaotic system very likely, which could result on EMC problems, output oscillation problems, or even the destruction of the modules themselves or the attached circuitry. 

Some modules/regulators have been designed to be ganged together. These will normally have a “share” connection that allows their control algorithms to cooperate for equally sharing the load. Others can be made to cooperate with a vendor-approved method that is provided in their datasheets.