The reason that its a bad idea is commented on in the text of your link. Variations in voltage regulation between the different regulator chips will lead to one chip passing more current than the other one. That eventually, will most likely cause failure of that chip, that in turn will mean that the other chip will carry all the load and will promptly fail.
as the others have suggested use a current pass transistor, a higher current rated reg chip, or even better yet, use a DC-DC buck converter chip they are so so much more efficient and as a result dont run very hot like the LM78xx series.
Dave