I have been using the same vaporizer for both. You just gotta be aware that any thc or dmt stuck in the vaporizer will interfere with the other.
DMT + THC will very strongly potentiate so do be careful especially if you are sensitive to any of them. You should properly clean it out and then test it empty if there is residual dmt or thc left before you go for the other drug. Usually once after clean i dry heat it to its max temperature an blow air through it to vaporize anything thats left and to remove the cleaning solvent. (I use drinking quality EtOH since it gets rid of both DMT and THC as a solvent)
Depending on the build of the vaporizer it might not be possible to clean it fully.
My vaporizer is a flowermate hybrid x. I use the ceramic chamber with that oil drum for dmt. Pretty much no dmt get stuck "in transit" but in the mouth piece, but since that is swappable i just have a DMT mouthpiece and a THC mouthpiece.