How to send multiple signals to a DAC
A sample rate of 1024 Hz is very low. You will need to filter the output of your DAC with a low-pass reconstruction filter that has very little response (at least 30dB of attenuation) above 500 Hz in order to eliminate the "image" frequencies created by the sampling process.

Suppose you were trying to generate tones at 100 Hz and 150 Hz. The raw output of your DAC will also have frequency components at


1024 - 150 = 874 Hz
1024 - 100 = 924 Hz
1024 + 100 = 1124 Hz
1024 + 150 = 1174 Hz


in additional to higher-order components. These additional components are not harmonically related to your original tones, so you perceive them as "noise".

Normally, a DAC for audio applications would have a sampling rate about 10x to 100x the rate you're using, with a matching output filter. Commonly used sample rates include:


8 kHz (audio up to 3400 Hz) - called "voice grade" because this is used for telephone circuits.
16 kHz (audio up to 7500 Hz) - AM radio grade
32 kHz (audio up to 15 kHz) - FM radio grade
44.1 kHz (audio up to 20 kHz) - "CD quality"
48 to 192 kHz - professional audio rates