the better way is to use switch mode regulator at such high loads. if you want to continue with linear, the way is to use series pass transistors with one regulating element
see the datasheet for 7812 at ALLDATASHEET.COM - Datasheet search site, Datasheet search site for Electronic Components and Semiconductors and other semiconductors.
PS: i understand that you already gave a datasheet
with linear regulation with 8V differential across, it really smokes at 20 amps
parallel working is problematic as you cant equalize the load across the number of elements
This is in general a bad idea as one regulator tends to take most of the load itself and therefore will burn first. Thereafter the other will probably burns one at a time.
If you had read through the datasheet, you would have found this method to boost output power: