Well ignoring the harp players... I once would have thought concertina and accordion an improbable combination, but ever since Tony MacMahon and Noel Hill's fabulous recordings, the number of albums featuring tina/squeezebox duos is growing steadily.
Now there's a new one from Jonathan Roche and Holly Geraghty. details and samples at http://www.hollygeraghty.com or http://www.jonathanroche.com
Sounds really lovely to me!
Other concertina and box duet albums:
MacMahon & Hill (In Knocknagree & Aislingí Ceoil)
Hugh & Colm Healy (Macalla Na hÓige)
Lonely Stranded Band
Mary & Andrew Mac Namara (Open Hearth)
Any more?