Can I connect multiple sources to an external DAC

Thank you all. Time for some more research and hopefully some auditioning.

Let's start by defining what your problem is first (and then you can correct me if I m wrong!) 

You have 2 digital sources and you want to improve the sound quality of both by adding an external DAC. otherwise there would be no need for adding an external DAC right?

if you buy a DAC that accepts 2 inputs (mine accepts Optical and Coax, but it does not matter ) the problem is solved that is because you are not going to be listening to 2 sources simultaneously. Just turn off the source that you are not going to listen to each listening session. Your DAC will lock to the incoming signal and will convert that one feeding the analogue signal to the preamp.

If you have more than 2 digital sources then a switching box such as the LINKFOR suggested above is the solution. 

Exactly, sound improvement and normalization if I can call it that. I'd like both sources to have the same quality. From all the comments, it seems like there are many options for external DACs that can switch between sources. 

Correct -- both the Topping and Schiit examples I gave have a button on the front panel to switch between inputs.  There are some DACs that "autoswitch" based on which input is providing a signal. In your case, that could be confusing to the DAC if both sources were playing at the same time.

  I think it's all been covered, but it doesn't matter that the DAC gas only one output to the preamp.  You can connect multiple digital sources to a DAC, but only play one of them at a time