It seems to me that the only way that someone could safely adopt someone else totem would be for both the following to be true:
- To have your own totem, which you can first use to confirm that you are not in a dream
- For you to totally trust the person you have taken the totem from, or that they are no longer alive
The second rule is just the normal "don't let anyone else know your totem's secret' - so really the only new thing here is the step of using one totem to confirm reality before adopting and learning the secret of a new one. From the moment you start shared dreaming ... your current totem(s) is(are) holding your reference to reality, and you should use a 'chain of custody' approach when transferring this reference to another device.
If you can do this, there is no reason why you could not safely use another totem - its just another physical object who's secret behavior only you (or you and the trusted other person) know.
Does it add credibility to the question that Cobb may not be dealing with reality any more? Unfortunately we are not shown how he adopts the totem from Mal, or whether he uses his original totem to confirm reality before he does so - but I would imagine that he does indeed do this. One can presume that this happens after Mal commits suicide, in a place that we know he believes is reality.
That said, the spinning top either has another secret that we as the viewer are not aware of, or it is a terrible totem as its secret is far too easy to understand. But I've talked about this in other Inceptions questions so I won't repeat that here as it does not add to the answer much.