Questions of this kind are especially difficult to answer as we have a new standards regime since the middle of 2012. Exactly how it will work out in practice is currently uncertain. You would need to take professional advice to get anything approaching a definitive opinion.
The new regulations involve a criminal offence in relation to any improper pecuniary interest. If councillors are trustees of a property not owned by the town council, then I would think that they should have recorded this in the register of interests. However, in the circumstances described, it doesn't appear as if the councillors have any relevant pecuniary interest. So my impression is that there is unlikely to be a criminal offence.
Other principles are much harder to enforce, and include stipulations such as "Holders of public office should not place themselves under any financial or other obligation to outside individuals or organisations that might seek to influence them in the performance of their official duties." While that principle could be relevant, its generality means that interpretation is liable to be subjective.
It does seem possible that the councillors could argue that they were working in the best interests of both properties, and only exact details of the circumstances could decide the matter.
