This is not the best way to increase the overall resolution. Combining two signals will introduce more noise, glitch, jitter and will affect the linearity of the DACs and resulted in an inaccurate signal.
For NI DAQ devices, only code width
can be manipulated to determine the step precision of a DAC inside the DAQ device.
User can only opt for another model with higher DAC resolution per channel, such as NI PXIe-4463
which have analog output channels with 24-bits resolution DAC each.