You may do it manually for the second and succeeding citations by editing the field.
Right click on the citation and select the "Edit Field" option.
For example, if the original citation was this:
(Johnson, Meyer, Lewis, Czerny, & Carroll, 2013)
The citation field would be something like this:
CITATION John13 \l 1033
If you edit it to this:
CITATION John13 \n \t \l 1033 \f "Johnson, et al.,"
It becomes:
(Johnson, et al., 2013)