Hey Justin!
Those 2 DAC pins should be independent.
External DAC seems like the way to go. I have used Adafruit MCP4728 Quad DAC with Arduino, and it seems to be working well. Here’s an entire video I made about it (4 CVs From Arduino! (DIY Eurorack Tutorial) - YouTube)
I have been meaning to use that with Daisy for a while now. I will look more into it and try it out; there seems to be past forum posts about I2C and Oopsy.
There is another approach to adding DAC and that is to use the PWM output of Daisy and running that into a lowpass filter circuit, which actually sounds easier than you think. It won’t be pristine clean but it gets the job done (especially for non-pitch CVs).
I always reference this video for that WERKSTATT-01 | LFO Quantizer Mod - YouTube. Just one resistor and one capacitor!
I have used this personally to make a custom eurorack controller 
Best of luck!