If I understand what you are asking, here is the basic syntax:
set term png size 600,400
set output 'plot.png'
set multiplot # engage multiplot mode
set size 1.0,1.0 # sets the size of the first plot
set xdata time ## these three lines control how gnuplot
set timefmt '%H:%M:%S' ## reads and writes time-formatted data.
set format x '%H:%M:%S' ##
plot 'data1' u 1:2 # plot the first data set
set size 0.4,0.4 # set the size of the second plot in plot units
set origin 0.15,0.5 # set the origin for the second plot in plot units
plot 'data2' u 1:2 # plot the second data set
This will plot the second data set as a subfigure.

To make two plots in a grid, you can use set multiplot layout:
set term png size 600,300
set output 'plot.png'
set multiplot layout 1,2 # engage multiplot mode
#set size 1.0,1.0 # sets the size of the first plot
set xdata time ## these three lines control how gnuplot
set timefmt '%H:%M:%S' ## reads and writes time-formatted data.
set format x '%H:%M:%S' ##
set xtics 120 # make time spacing of 2 minutes
plot 'data1' u 1:2 # plot the first data set
#set size 0.4,0.4 # set the size of the second plot in plot units
#set origin 0.15,0.5 # set the origin for the second plot
plot 'data2' u 1:2 # plot the second data set
unset multiplot
