What is the term for an art show featuring two artists?
A "group exposition" or "collective exhibition" could be used to describe a two person art show ... BUT (after searching the internuts a bit) it seems that "Duo Exhibition" is the most common term for a two artist show. Group Exposition or Collective Exhibition applies to shows which have three artists or more. 

Examples:


In the exhibition guidelines for the MacDonald Island Community Art Gallery Exhibition they list three categories of exhibitions which depends on the number of artists showing: Solo, Duo and Collective
SEÇİL BÜYÜKKAN gallery discussing the Seçil Büyükkan and Deniz Aktaş "Hande Şarman" show  refers to the event as a "Duo Exhbibition".
The CV for visual artist Yooree Yang gives a listing of Solo Exhibition &amp; "Duo Exhibitions" the artist has participated in the past