I have previously mixed two sines to create DTMF. Simply add the two signed values together before pushing to the DAC does the job quite nicely. I was using a DAC6 @ 10kHz for this task, then feeding through a LPF in 2 analog blocks nearby before feeding the output to the phone line.
Thing to note is be make sure that adding the two sines together does NOT cause overflow, otherwise you will get all sorts of harmonics, making it sound awful and failing the DTMF. Pick the two maximum and minimum values, add them together and make sure does not overflow the DAC (+31->-31 for DAC6)
My new project I am using DAC8 @ 8kHz, but same configuration DAC8 -> LPF -> PGA -> Phone Line.
Examine the DTMF module in the designer for another information source. Also, there was an appnote on a polyphonic piano that used the MAC to scale and combine up to 10 notes before playing them - neat! Search Cypress for "polyphonic".