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.