In this case, following the usual configuration instructions, you would have an individual membership configuration in the web application's web config. You would have all of the membership configurations in the central admin web.config and token provider
What I usually do is put all of my membership configurations in the machine.config file (C:\Windows\Microsoft.NET\Framework64\v2.0.50727\CONFIG). Then you don't have to copy them into each individual web.config. Unfortunately you still have to
put them all in the token provider web.config, as it overrides the machine.config at some point.
I haven't tested this with 2013 yet (on my todo list!), but this user reports that it seems to work: