http://www.aaroncake.net/circuits/volume.asp
http://datasheets.maxim-ic.com/en/ds/DS1669.pdf
I do not know a lot concerning the circuit provided since i have yet to experiment with the IC but the above links may provide the info you are looking for; appears simple enough.
When used in a stereo mode i imagine you would use two of the ICs along with two DPST N.O momentary push button switches.
Although, it may be easier along with less expensive to locate 2 DPDT momentary pushbutton switches and only use half the switch etc...
You could also Google the IC part number where more circuit design may be available.