BCECF AM is intrinsically nonfluorescent until it is cleaved by
intracellular esterases to BCECF, which is a green fluorescent
fluorescein derivative. The 5-6 negative charges on BCECF cause it to
be retained in cells that have intact membranes for up to a few hours.
On the other part of your question: I presume that the cells would be
loaded with BCECF AM (about 30 min with 1 micromolar reagent), washed
and then comixed with the other cells.
You can find an extensive bibliography on BCECF AM (cat #B-1150) at
Molecular Probes' Web site (http://www.probes.com.