Use the Grouped Reference option to cite with one number any references that always appear together as multiple citations. To locate the Grouped Reference option for the desired output style, go to Edit > Output Style > Selected Output Style > Citations > Numbering.
One number is used both in the text and the bibliography to identify a group of citations. There is an option to specify how the grouped references in the bibliography should be separated.
This option shows a semicolon and a space as a separator:
1. McCormick, MP, Geophysical Research Letters (1999) 15, 907; Pemberton, SG, Jones, B, Journal of Paleontology (1999) 62, 495; Postma, G, Sedimentary Geology (1999) 58, 47.
To differentiate individual citations within grouped references with letters (see above):
Define a composite citation:
1. Select the box to “Use letters to separate grouped references.”
2. Use the “Present letters in” list to select Lowercase or Uppercase letters.
3. Select the appropriate letter format for in-text formatted citations. The examples illustrate how letters and numbers appear for consecutive citations and ranges.
4. Use the “Precede by” and “Follow by” boxes to enter any character(s) chosen to surround the letter in the bibliography.