I tried that as well, but no joy.

This is certainly what you want.

import SweetXml

"""
<complexTypeTwo xsi:type="ns1:ComplexTypeOne" xmlns:xsi="http://www.w3.org/2001/XMLSchema">
    <identifier xmlns="urn:bar">Two</identifier>
    <some>thing</some>
    <other>stuff</other>
</complexTypeTwo>
"""
|> parse(namespace_conformant: true)
|> xmap([
   complex_base_type: ~x[/complexTypeTwo/@foo:type]s |> add_namespace("foo", "http://www.w3.org/2001/XMLSchema"),
   identifier: ~x[bar:identifier/text()]ls  |> add_namespace("bar", "urn:bar"),
   some: ~x[some/text()]ls,
   other: ~x[other/text()]ls,
])

You cannot ‘just’ say you want xsi:type without saying which XML namespace the xsi prefix corresponds to. The actual source doc from your sample wasn’t namespace compliant in the first place. You see the xmlns:xsi="..." binding at the root elem.