[R] Plotting multiple layers(maps) on same page

[LCOG1]

Hey guys and gals,
   I searched through the forum and a bunch of R-mapping dedicated sites but
have not found what i know is quite elementary process, mapping more than
one layer on the same plot.  I need to show some reference lines for the map
to make sense.  I know the below wont work for anyone but to show what im
trying to do:

TazFile <- "data/input/TAZ.shp"
UGBFile<- "data/input/metugb.shp"
TazShape <- readShapeSpatial(TazFile)
UGBShape <- readShapeSpatial(UGBFile)

#Write out to pdf
pdf("Results/HhEmpForecast.pdf", width=8.2, height=11, onefile=TRUE)
#Households
plot(TazShape, col=colsHh[findInterval(TazShape$Hh, brksHh,
all.inside=TRUE)])
plot(UGBShape)
title(main="Households Forecast by Taz")
legend(4199277,860791.2,title="Total Households by
Taz",LegBrksHh,fill=colsHh,cex=.8)

#Employment
plot(TazShape, col=colsEmp[findInterval(TazShape$Emp, brksEmp,
all.inside=TRUE)])
title(main="Employment Forecast by Taz")
legend(4199277,860791.2,title="Total Employment by
Taz",LegBrksEmp,fill=colsEmp,cex=.8)
#Close file
dev.off()

I need Households and employment on separate maps but i need the UGB file to
be on each for reference. Doing it the above way obviously puts the
reference map (UGB) on a separate map.  So how do i plot them on the same
page.  I thought this was easy enough but im not finding a simple answer. 
Thanks for the help.  

In Solidarity, 
JR
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