[R] One main caption for an mfrow=c(2,1) plot window

[Alexander Engelhardt]

Hello,
I'm trying to plot a series of pages in a pdf with one main caption for 
each page.
Each page should then have two plots in one row, preferably with an own 
caption.
I can't plot the main caption in a window, and subsequently plot the two 
graphics (next to each other) below it.

Here is what I've tried. This one almost does the job, but when plotting 
the two graphics, R generates a new pdf page.
The 3 commented lines were another approach I tried but didn't work out. 
The split.screen() erases what was previously drawn on the device (which 
is the main caption in my case):

x <- 1:10
y <- 10:1
pdf("sandbox.pdf", width=6.5, height=3)
for(i in 1:3){
   plot.new()
   title("derp")
   op <- par(mfrow=c(1,2))
#  split.screen(c(1,2))
   plot(x)
#  screen(2)
   plot(y)
#  close.screen(c(1,2))
   par(op)
}
dev.off()

Any ideas?

Thanks in advance,
  Alex