This software (MG2/MG3) does not recognize what string you are using for your pitches, so there are no per-string assignments of any kind available at this point. You can however decide to treat a group of pitches to some special treatment.
As @LoFiLeiF mentioned it doesn’t distinguish among strings, but for this case it the PITCH FILTER may do the trick.You could go further and add a 3 way split and add another TRANSPOSER or the BASS FILTER or different synths for the splits. The idea with MG3 is to make such compositions of effects easy.