I am not familiar with that device, but, in general when paralleling voltage output devices which were not specifically designed to be paralleled you need to include some ballast resistors. They allow for slight differences in the output voltages of the two (or more) devices and promote current sharing.
Put one resistor in series with each output. Connect the other ends of the resistors together and connect this to the load. The value of the resistors depends on the amount of current, the voltage difference between the two outputs when given the same digital code, how much voltage drop in the resistors can be allowed, and perhaps other factors such as how fast the output must change.
I can help with the calculations if you are not familiar with them, but I will need more information about the voltages and loads.
Lynn
