You can use both, but I'm not sure if there is an advantage to or not as most .ini configurations are made specifically for running one or the other alone. I however liked the SMAA Antialiasing option in SweetFX for which didn't appear to be present in ENB Series. Fortunately ENB Series allows you to inject another d3d9.dll with it. So I downloaded the InjectSMAA standalone .ini with it's d3d9.dll file and what you can do is rename that file to say SMAAd3d9.dll and open the enbseries.ini and at the top there is a section called [PROXY] and set it to the following:
[PROXY] EnableProxyLibrary=1 InitProxyFunctions=1 ProxyLibrary=SMAAd3d9.dll
Of course that dll file can be any other DirectX injector DLL that you would want to try alongside ENB Series. I'm sure I could have got the same result with the SweetFX DLL with only the SMAA enabled within it.