  Microlensing is increasingly gaining recognition as a powerful method for the
detection and characterization of extra-solar planetary systems. Naively, one
might expect that the probability of detecting the influence of more than one
planet on any single microlensing light curve would be small. Recently,
however, Griest & Safizadeh (1998) have shown that, for a subset of events,
those with minimum impact parameter $u_{min} \lsim 0.1$ (high magnification
events), the detection probability is nearly 100% for Jovian mass planets with
projected separations in the range 0.6--1.6 of the primary Einstein ring radius
$R_E$, and remains substantial outside this zone. In this Letter, we point out
that this result implies that, regardless of orientation, all Jovian mass
planets with separations near 0.6--1.6$R_E$ dramatically affect the central
region of the magnification pattern, and thus have a significant probability of
being detected (or ruled out) in high magnification events. The probability,
averaged over all orbital phases and inclination angles, of two planets having
projected separations within 0.6--$1.6R_E$ is substantial: 1-15% for two
planets with the intrinsic orbital separations of Jupiter and Saturn orbiting
around 0.3--1.0$M_\odot$ parent stars. We illustrate by example the complicated
magnification patterns and light curves that can result when two planets are
present, and discuss possible implications of our result on detection
efficiencies and the ability to discriminate between multiple and single
planets in high magnification events.
