Multiple colorbars in gnuplot

Question

I'm trying to plot two data series in the same graph with multiplot. I would like each of the two data series to be plotted with a different palette. So far I've been trying with multiplot,

set multiplot layout 1,1
#
set palette rgb 21,22,23
splot "S1"    using ($1):($2):($3):($5)  with points pt 7 ps 3 palette notitle, \
      "S1"    using ($1):($2):($3):($5)  with impulses linecolor "black" notitle
#
set palette rgb 23,28,3
splot "S1" using ($1):($2):($3):($5)     with points pt 7 ps 3 palette notitle, \
      "S2" using ($1):($2):($3):($5)     with impulses linecolor "black" notitle
#     
unset multiplot

however, I can not visualize in the X11 terminal the two series at the same time. Any idea?