Can I connect multiple sources to an external DAC

Depends on the DAC.  Some have multiple inputs with a front panel switch to choose the source.  Most commonly you'd see a coax input, an optical input and a USB input with a switch to choose between them.  If your CD and Node are capable of different type outputs this would solve your problem.  There are fewer DACs that have two coax or two opticals inputs. If your sources only have one output and both are of the same type you'll need one of these DACs which will limit your options.  You'll need to figure out your needs and then research the market for what's available.

Depends on your DAC. 

Thank you. In shopping around, I could see the multiple inputs but didn't see a way to switch. Good to know that it exists. Both sources have optical and coax.

Two examples of DACs with multiple inputs (I've owned both.) At the relatively inexpensive end, a Topping E-30 has coax, optical and USB inputs which are switchable from a front button. The Schiit Bifrost at $800 also has these inputs which are switchable from a front panel button. Other Schiit and Topping Dacs can also do this, but check the details before you buy. 

Thank you. The Bifrost is a contender for sure.