Production regulators have tolerances - in the order of a couple of percentage points (most are 3 or 5%).
So, a 5V regulator might be giving 5.15V (5+3%) and the other one 5.00V. If you parallel them, the one providing the load is the highest one. The lower still thinks that the voltage is too high and waits for it to drop.
Until the highest regulator gives up (its protection circuitry kicks in), it will provide all the current (and heat up). I used an extreme example of difference in voltage between two regulators. In practice they vary less. On the other hand, just a couple of mV (millivolts) is sufficient to unbalance the load carried by each
So, putting two regulators in parallel is not a good idea.
There are circuits on the 'net which connect an external transistor to boost the output of a single regulator. It's slightly tricky because, unless you calculate a couple of resistors really well, the protection circuit won't work well. And connecting an extra transistor usually means the voltage drop across the regulator has to be at least 0.7V more.