When you play 2 instruments together, it is always called a duet, no matter what instrument family they are from. Hope that helped! :)
A "classical band of four" is called a quintet, and traditionally uses two violins, a viola and a cello. For two, you can use any two string instruments.
two violins, viola and cello
Two violins,viola and violoncello
They use the same instruments as all string quartets: two violins, one viola, and one cello.
Two violins, a viola, and a cello.
string instruments
There are four types of musical instruments: String, Wind, Brass and Percussion.
Bowed and plucked
String instruments sit at the front of the orchestra because of the volume/intensity of the sound that they are able to produce compared to the brass, woodwind, and percussion. If the strings were to be placed behind the band, then the audience would have a hard time hearing the string instruments, because band instruments generally produce louder sounds than string instruments. It is because of the volume balance that sometimes even the string instruments are rearranged. In example, sometimes the violas and the cellos switch places because the cellos play to softly, or because the violas play too loudly, or because of both.
These are called "stringed instruments," also known in the Hornbostel-Sachs organology as chordophones.
Usually, two violins, a viola, and a cello.
All string quartets are made up of two violins, a viola and a cello.