well, theoretically may be its possible, i guess there are some circuits, but if there is difference in their regulated output voltage even by a slight, the problem can arise. For eg. one regulator gives 5.1V and another 5.2V, the regulator with 5.1V output will do most of work, overheat and shut the regulator off.
So, instead of directly connecting the two outputs, may be you can connect them through two different output resistor of high wattage and having different feedback circuitry for both of them.
But, ideally, i suggest using single regulator with current and voltage rating you need. U can build a small microcontroller based power supply, there are many application notes from microchip.
Hope that helps.