What often happens is that the two supplies do not share well. (two power supplies, 10A each)
Power supply 1 outputs 5.000 volts.
Power supply 2 outputs 5.005 volts.
When connected in parallel power supply 2 will output 5.005V when the load is 0 to 10A and supply 1 will not output.
At 11A of load supply 2 will output 10A and supply 1 will output 1A and the voltage will be 5.000V.
To avoid the issues described by ronsimpson , You must add a "shock absorbing" resistor in series with each power supply before you parallel them. To calculate the value of the shock absorbing resistor, determine the variability under load and The maximum current of each power supply at your full load.
the resistor value will = voltage variability / Max load current.
This means your available voltage will be somewhat less than the specified voltage of the power supplies. In the end, you'll be much better off buying one appropriately sized SMPS for your project rather than paralleling several of them to make more current.
They have to be designed to do so. basically a master/slave arrangement. Search for the "HP power supply handbook".
Simplistic explanation: Two identical 5V 5A supplies. one would be set up to deliver slightly less than 5V @ 1/2 the constant current value. The other supply would be set up to deliver constant current with a max voltage of 5V and a max current of 1/2 the desired.