Note that at the time, the very term "holster" pistol implied that it would be carried in a saddle (pommel) holster, rather than on the person. Pistols to be carried on the person would be called "belt" pistols. Pommel holsters carried pistols in pairs, one on each side.
Also, because of the difficulty in reloading Paterson pistols, and their tendency to foul, often to the point of jamming, after discharging a single cylinder, it would make sense to carry a pair.
Purchasers of these guns, particularly for use on the frontier, would have liked to buy two of them. And probably, a lot were sold in pairs. ("A lot" is a relative term. In absolute terms, not many Patersons were sold. This led to the bankruptcy of the firm.)
Whether these were sold in cased sets is a separate question entirely. The cases would have been important for presentation purposes, and for wealthy individuals, but not very much for actual users.