The Party Wall etc. Act allows for one surveyor to represent both parties to a dispute. (i.e. the Building Owner and the Adjoining Owner). The party wall surveyor should be fair and honest at all times to properly represent both their interests.
This arrangement can work well with straight forward matters and when the neighbours get on well — and especially when the matter is non contentious. If a party wall surveyor sees issues that are likely to be problematic or contentious, it is best to decline an agreed surveyor role.
As the Act enables and allows the parties to the dispute to agree that one surveyor will represent them both this option should be considered favourably as this can be a cost effective and usually a quick way of dealing with straight forward and non-contentious matters. However, both parties have to agree and equally the surveyor must be comfortable that the matter allows for such with no potential for a conflict of interest.