Hey there, great question! And an excellent candidate for using the arcpy.mapping module.
The first step would be to create a list of the mxds you want to modify. Something like
import arcpy
arcpy.env.workspace = r"[path to directory containing your MXDs]"
mapDocList = arcpy.ListFiles("*.mxd")
Now that we have that list, you can identify the extent you want to apply to the other MXDs. Try:
mxd_to_copy = arcpy.mapping.MapDocument(r"[path to MXD containing the extent you want to copy]")
df_to_copy = arcpy.ListDataFrames(mxd)[0] # Note: this assumes that there is only one data frame in the mxd. If there are multiple, you can specifiy which to use with a wildcard
extent_to_copy = df_to_copy.extent
Ok, now you have created an extent object that you will apply to the data frames in your other MXDs. So...
for mapDoc in mapDocList:
# Create an mxd object out of the map doc
mxd = arcpy.mapping.MapDocument(r"[path to directory containing your MXDs]" + "\\" + mapDoc)
# Create a data frame object to apply the extent to
df = arcpy.mapping.ListDataFrames(mxd)[0] # See previous note about multiple data frames
# Apply the extent
df.extent = extent_to_copy
# Save the map document
mxd.save()
# Cleanup the variables
del mxd, df
Anyways, the arcpy.mapping module is awesome once you get the hang of it. Esri really hit it out of the park with that one in my humble opinion.
I hope this works!
Warm Regards,
Micah