Unless you modify the circuits, it's going to be hard to parallel these without losing a bit of power. The problem is their voltage will not be 100% the same, so if one is 13.80V and the other is 13.81V, the later one will be taking much more of the amperage that the load is sinking.
A way to overcome this is to place a very low value resistor in series with the outputs of all the DC-DC converters. When one of them is significantly above, the extra current though the resistor will generate a drop in voltage, which means the output voltage of that module will eventually match the rest.
And try not to run these at 2A. They seem quite small in my opinion and although they claim they can handle 2A, I guess they will be happier when ran at 1.5A, specially in a demanding environment like a motorcycle, with the heat and vibration.