I would try using the xlim, ylim arguments in your first map() command, and then using the add argument in the second one. See the help page for the map function.
This assumes that the map() function you are using comes from the maps package.
Otherwise, perhaps you should be using the rgdal package to load your shapefiles into R (you did mention shapefiles), and the basic plot() command to plot them. Depending on whether you choose to use the 'sp' package or the 'sf' package.
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