Yeah its a bad idea, I havent read the other responses but my quick opinion is.
Your both there because you have a drug problem... So that means your experiencing some dysfunction and instability... both of you.
If you mean for a longterm thing like meet someone and be together once your out, it never works, I've been in treatment a good bit, and observing others and my experience hardly anybody really stays together once their out.
To have a successful relationship, 1st you have to love and respect yourself, then know your independent and comfortable in life...
As for a no strings attached thing, sure it could work, still you may find yourself stuck on the person, or wanting to get rid of them, and that would be hard because you'd be living at the same place. Plus you can easily get kicked out and most relapse at that point, I did. ha.... anywho.... Its just a sketch situation... defiantly if you wanna get down while your there just be careful and do some digging as to the in's and out's of whats possible to get away with.
Oh yeah also if your serious about recovery there is no way in hell this is a good idea at all. All of your focus would be on your rehab romance and not on the valuable information your there for.