Combining LDOs in an effort to increase current
Tying multiple LDO's together is a bad idea.  No two are going to have exactly the same output voltage regulation point.  One will take more of the load than the other, possibly stressing it more than it should be.

No, you can't assume you know the temperature response of a regulator only because you think you know something about the temperature response of its final pass element.  There is a whole feedback and control circuit in there, quite likely with temperature compensation.  Unless the datasheet explicitly tells you, you can't make any assumptions about the output voltage regulation point as a function of regulator temperature.

Some regulators have internal thermal shutdown which will prevent damage due to over current, but that's not going to be good for maintaining the output voltage.  When one shuts down, the others have to pick up the slack.  If you're lucky, the first one will come back on before the rest shut down, but that's hard to guarantee.  Even if enough always do stay on, the output supply will likely be glitched at the on/off transitions.

Even if the regulators have enough softness in their output voltages so that others will share the current, it's still not a good idea because each has its own controller, and those controllers will be fighting.  You could quite possible get oscillation of the output voltage.

The simplest way to make multiple regulators share current is to put a small resistance in series with each.  For example, separate 100 m&Omega; after each regulator, then the outputs of those tied together might work.  Make sure each regulator has its own output cap, which must be immediately at the regulator before the resistor.  The downside of this is that the DC resistance of the final supply will be higher than before.  Whether that's OK or not is up to you to decide.  If you tie 4 regulators together, each with their own 100 m&Omega; resistor, then together they only add 25 m&Omega; to the impedance of the regulated line.  That would be good enough in many cases.

Depending on how low your headroom voltage really is, this standard trick for making a high current regulator from a low current one might work:



This adds the B-E drop of the transistor to the headroom requirement of the regulator.

As the current thru the regulator increases, the voltage on R1 increases proportionally.  Eventually the voltage on R1 gets high enough to turn on Q1, which then starts shunting current around the regulator directly from the input to the output.  The maximum current thru the regulator will be the B-E drop of Q1 divided by R1, which should be about &frac34; A in this example.