You CANNOT directly place three-terminal regulators in parallel for a higher current output. Regulators have a voltage tolerance (±0.1v is not unusual) and no two regulators regulate at exactly the same voltage. If you place two directly in parallel and one regulates at a voltage that is 0.05v more than the other, this regulator will do all it can to bring the output voltage up to what it wants while the other sees the higher voltage at the output and shuts down, leaving all the current load on the first device.
One way around this problem is to parallel the chips' input and ground, but place a low-value (e.g., 0.1 ohm) resistor in series with each output terminal before connecting them together at the other end of the added resistors. This will provide a resistance for the small difference in regulation voltage between the two at the sacrifice of making the supply just a wee bit "softer", i.e., not being able to regulate as tightly as a single chip. Usually this is not a big problem in most applications.
Checking out the 7805 data sheet will show methods of using extra parts around the regulator for extra current capacity. Also, the TO-3 7805K package will handle 1.5 amps when properly heat sinked.
Don't forget that the higher your raw dc voltage to the chip, the more it will dissipate, so keep the raw dc to a just above the necessary headroom for best regulation, especially on higher current supplies.
Dean