From a normative point of view, RFC5280 does not put any constraint on having both extended key usages set at the same certificate.
From the security point of view, there is also no crypto/protocol problem (as far as I know/could find) on using the same certificate for SSL authentication as client (I guess for connecting to other services) and server (for receiving connections). Since if you use two certificates both would also be stored on the same server, if an attacker can steal one certificates he can steal two, no gain or loss there.
However, it does not hurt to keep them separate too, specially if for any reason later on you need to change one the certificates characteristics in a way that could affect the functionality on one of the usages (ex: change the DN to include something relevant to the client auth that could break the server auth).
I always, in such situatins, keep both separate, and if you are talking about private PKIs, usually I also use different CAs for issuing each of them.