gistfile1.txt

import xml.etree.ElementTree as ET
from xml.dom import minidom

namespaces = {
    'sld': 'http://www.opengis.net/sld',
    'ogc': 'http://www.opengis.net/ogc',
    'gml':'http://www.opengis.net/gml'
}

for prefix, uri in namespaces.iteritems():
    ET.register_namespace(prefix, uri)

filenames = ['SLD1.txt', 'SLD2.txt']

with open('ConcatenatedSLDs.sld', 'w') as outfile:
    out_root = ET.Element("sld:StyledLayerDescriptor")
    nl = ET.Element("sld:NamedLayer")
    out_root.append(nl)
    name = ET.Element("sld:Name")
    name.text = "Some Style Name Here"
    nl.append(name)
    us = ET.Element("sld:UserStyle")
    nl.append(us)
    title = ET.Element("sld:Title")
    title.text = "Some Title Here"
    us_name = ET.Element("sld:Name")
    us_name.text = "Some Name Here"
    us.append(title)
    us.append(us_name)
    fts = ET.Element("sld:FeatureTypeStyle")
    us.append(fts)
    fts_name = ET.Element("sld:Name")
    fts_name.text = "Some Feature Type Style Name"
    fts.append(fts_name)

    for fname in filenames:
        with open(fname) as infile:
            root = ET.fromstring(infile.read())
            rules = root.findall('./sld:NamedLayer/sld:UserStyle/sld:FeatureTypeStyle/sld:Rule', namespaces=namespaces)
            for rule in rules:
                fts.append(rule)

    output = ET.tostring(out_root)
    reparsed = minidom.parseString(output)
    outfile.write(reparsed.toprettyxml(indent="  "))