We already allow secondo, "The second or lower part in a duet" (online Oxford). Secondo is known to most dictionaries, whereas I couldn't find secundo in any, other than Wiktionary. Secondo is from the Italian, the source of most musical terms. Secundo is Latin.
Is secundo really a word in English? It's true you can find web sites where people use it for the second part in a duet, but maybe they're just poor spellers. Sorry, ridethetalk, but I'm not convinced.