Once you have read both time series in GrADS, then you can easily plot both, each at a time, by using the "display" function. You should first take the two variables to the same units (maybe mm/day) in order to ease comparison. In this sense, take into account that mm and Kg/m2 are equivalent units -- so you should only pay attention to have both in the same time units.

It would look something like:

>> set ccolor 1

>> d precip

>> set ccolor 2

>> d precip.2

Thank you Ramiro Saurral