Not sure what the professional ethics are in this regard. But they're paid shills, so everyone knows it's bovine excrement anyway
More seriously, I doubt if there's such an obvious conflict of interest as, for example, in a local government context where decisions can more directly affect people and business, or in a banking environment, say, where different financial interests can come into conflict*.
Or, more obviously, in a legal environment, where a firm of lawyers could be acting for two different sides in a litigation suit. Which seems to be allowed, as long as, say, confidential information isn't shared, and the same people aren't involved in representing both sides
But I suspect that in the PR environment, they've probably got different teams dealing with the pro-LEZ and the anti-LEZ stuff, and to that extent as long as they don't collude then there's probably no professional ethics issues.
(*What used to be called 'Chinese walls' in a financial environment - if there's a conflict of interest then obviously the people dealing with one interest wouldn't be allowed to swap notes with the people dealing with a competing interest. Although I think the term 'Chinese walls' is now considered racist, surprise, surprise
The
Wikipedia entry on Chinese Walls suggests alternative terms like 'ethical walls' or 'firewall'. )