A beautiful Minuet for Flute, Viola and Guitar by Carl Maria von Weber (1786-1826).
Five pages all instruments, two pages Flute part, two pages Viola part, two pages Guitar part, 11 pages total.
With downloadable mp3 for audio help., 4:30' min.

Mazurka op.6 No.1 by Frederic Chopin for piano and flute duet or piano and violin duet. Simplified piano.
With downloadable mp3 for audio help.
Pages: Seven pages (piano and flute 5 pages, Flute part 2 pages), 3' min.

Londonderry Air is an air that originated in county Londonderry, Northern Ireland.
Here is an arrangement for Flute and guitar duet. Early Intermediate.
With tablature. With downloadable mp3 just for audio help. 
With lyrics and melody line. With finger position suggestions.
Pages: four pages score and parts (3 pages both flute and guitar, 1 page flute part), 1:55 min.

Foggy Dew is the name of an Irish traditional song first appearing in Edward Bunting's The Ancient Music of Ireland (1840).
Here is an arrangement for Guitar and Voice or Flute and Guitar duet.
With lyrics and Melody line. With guitar standard notation and Tablature.
Four pages (three pages both guitar and flute, 1 page flute), 2' min.
With downloadable simple mp3 for audio help and practice the Flute. Easy.

12 Walzes for Flute and Guitar duet by Matteo (Mathieu) Bevilacqua (1772 - 22 January 1849).

For Flute and Guitar duet.
Early intermediate.  Guitar standard notation.

With downloadable mp3's just for audio help or practice (one with both flute and guitar, and one with guitar only).

Pages: Twenty five (25) pages, (1 page cover, 12 pages score with both Flute and Guitar, 6 pages Flute part,  6 pages Guitar part).