It's always worth remembering that what datasheets and application notes do not show can be just as important as what they do show.
In the case of simply paralleling up 780X regulators to get extra current capability, it would be logical that if this were possible the datasheets would say so and give an example of such use, especially when they describe other means to increase the current capability. The absence therefore suggests that this is not a suitable configuration or, if it could work, the manufacturer is not prepared to guarantee or recommend such operation ( eg, the caveats for such use are too numerous to make it worthwhile ).
Were a manufacturer being protectionist, hoping to sell a more capable, higher priced component, for common issues such as higher current regulating there would probably be some information on the Web saying it wasn't necessary and demonstrating the simpler, cheaper alternative.
So it's a pretty good bet that if the manufacturer doesn't say it can be done, no one on the web shows it can be, then it's likely not possible. That's not necessarily a reason not to try though if done in a proper and safe environment and you are prepared to accept damage or destruction of the components during research. It's always a good idea to ask what might happen before trying though, don't just dive in and win a Darwin Award.