Manual:$wgMemCachedServers

Array of memcached servers (for use with the memcached memory-based object store)

Details
To use multiple servers (physically separate boxes or multiple caches on one machine on a large-memory x86 box), just add more items to the array. To increase the weight of a server (say, because it has twice the memory of the others and you want to spread usage evenly), make its entry a subarray: More information in.