The Arrangement in the book:
The Arrangement was very simple, so simple in fact that it didn’t really deserve the capital letter, which it had got for simply being in existence for so long. It was the sort of sensible arrangement that many isolated agents, working in awkward conditions a long way from their superiors, reach with their opposite number when they realize that they have more in common with their immediate opponents than their remote allies. It meant a tacit non-interference in certain of each other’s activities. It made certain that while neither really won, also neither really lost, and both were able to demonstrate to their masters the great strides they were making against a cunning and well-informed adversary.
It meant that Crowley had been allowed to develop Manchester, while Aziraphale had a free hand in the whole of Shropshire. Crowley took Glasgow, Aziraphale had Edinburgh (neither claimed any responsibility for Milton Keynes, [Note for Americans and other aliens: Milton Keynes is a new city approximately halfway between London and Birmingham. It was built to be modern, efficient, healthy, and, all in all, a pleasant place to live. Many Britons find this amusing.] but both reported it as a success).
And then, of course, it had seemed even natural that they should, as it were, hold the fort for one another whenever common sense dictated. Both were of angel stock, after all. If one was going to Hull for a quick temptation, it made sense to nip across the city and carry out a standard brief moment of divine ecstasy. It’d get done anyway, and being sensible about it gave everyone more free time and cut down on expenses.
It’s the same in the book, they both do the blessing and the tempting if they are going to the same place (the last paragraph). Plus they don’t sabotage each other’s activites. :)