This will largely depend on two factors: 1) your divorce decree and 2) your ex-husband's position on the arrangement. If your divorce decree specifically bars cohabitation and your ex-husband is unhappy with your living situation to the degree that he's willing to reopen the case, then yes you could be in contempt of the court. The burden of proof would then shift to you to prove that the two of you are not living together as husband and wife (an intimate relationship), which is the definition of cohabitation: I can tell you that's going to be a difficult burden for you to prove simply because of the circumstances.
It is a fact that many, many people actually cohabitate post-divorce and the other spouse does not raise this ever in a court of law (my own parents actually both cohabitated with their current spouses at different times but it was never brought up in court, so it was not an issue!).
I would recommend reviewing your divorce decree thoroughly, and if you are seriously concerned I would discuss with your ex-husband (with whom you must get along to some extent, otherwise you would not be in a joint custody situation).
Legal disclaimer: DISCLAIMER: This response is for informational purposes only and does not form an attorney-client relationship. It represents only the educated opinion of the author. It should not be relied upon as legal advice. The response is based on the limited facts provided, and might change given additional or different facts.
If you get crossways with your ex, then based on my experience that could be a problem. I can depend upon the Judge more than anything else. However, in Arkansas I have found that most judges will take a dim view of any male roommate situation even if you have separate bedrooms.
This answer does not create a attorney client relationship and cannot be relied upon as a legal opinion as there are too many unknown facts to this matter to render an opinion at this time. Campbell & Grooms, PLLC http://www.campbellgrooms.com
A weekly guide with tips and legal advice for each stage of the process.