Hi,

The IC package can only dissipate a certain amount of heat.
If you use only one output the max current for the package is 480mA.
If you use two outputs, the current is 280 mA per output so the total
current for the package is 560 mA. This small difference is because
some of the heat is dissipated by the metal of the pin connected to
the PCB.

There is no need for a parallel connection.
Do not use a parallel connections with this IC.

Best regards,

Luciano