By what method did you ascertain that the nonpolar residue is the dmt and the evaporated water residue is the nmt?
Could you heve been creating nmt carbonate + dmt carbonate instead, until the carbonic acid in the soda wsa exhausted? In that case, you would not have achieved any separation, just a partial salting of the mixed alkaloids. In aqueous environment, carbonates are formed. Carbamates form in nonaqueous environments.
It would be nice if this proves to be a valid separation tek, by carbonate or carbamate. But there needs to be more analysis before anything can be said conclusively.