The government was always worried about revenues being cleaned and reused. This was especially true of the first issue revenues of 1862-1871, and was the reason that the second and third issues came out on the Willcox Chameleon Paper which was designed to prevent this problem.
Cut cancels were one of the more obvious solutions. They were devices which ranged in style from a simple 4 straight slices to circular cuts to actual punch holes of various types which removed parts of the design. Perforated initials were also used frequently, especially in the 20th century.
Pairs of these particular stamps are not very uncommon, although they are nice to have. Any revenue multiple is worth keeping in one's collection regardless of catalog value. Because revenue stamps were created to pay specific tax rates they usually had face values that matched the needed tax amounts. Therefore in general they were used singly much more frequently then in multiple, although for the most common types of taxes (especially from 1898 forward) multiples were used with great frequency.
This site is a great place to see what multiples are known to exist on many revenue stamps. Of course it is far from complete and should only be used as a guide, although the first, second, third, and 1871-1875 proprietary issues are a pretty fair indication of what actually exists.
http://www.thecurtiscollection.com/