A bit of a late reply but for posterity, if two people are on a lease then those two people have the right to use the property. They are legally responsible to pay the rent as well. Getting these tenants off the lease would only be accomplished with the landlord's assistance since the lease is with the landlord. If there is a clear violation of the lease terms then it may make things easier for the landlord to sever that relationship with the other tenants.
It is best to get notice or to keep a record that people who have stated that they are abandoning their residence are doing so. To protect yourself, you can send notice via some method that can be reproduced or saved, perhaps email, text messages, phone records or anything which you can at least show that you made an attempt to reach the person. It is usually best to have terms that state what is done with abandoned property. But before taking any action, such as selling or disposing of the property, you should show that you had made at least reasonable efforts to contact the prior tenants and that it appears rather certain that the items were abandoned. The higher the value of the items the more difficult this may be to prove, e.g. leaving a new Acura in the garage, etc.