If you have the means to provide a new animal its own enclosure, you should do definitely that. Cohabituating allows for quite a few issues without any real benefit (besides money, but these are animals we're talking!):

- spread of disease, if one animal gets sick, the other will get it too

- inability to tell who defecated, shed, peed, etc. If something looks unusual or someone regurged, who knows which bodily product came from which animal?

- competition of resources that causes stress -- one snake will usually lay on top of the other in struggle for good basking spots, like over the heat tape

- feeding stress - you can't feed them in the enclosure, so you have to separate them in different boxes, which causes unnecessary stress