Manual:$wgMemCachedServers

Array of Memcached servers (if you are using Memcached memory-based object store) Ignored if $wgUseMemCached is false

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:

$wgMemCachedServers = array(   "127.0.0.1:1100 0", # one gig on this box    array("192.168.0.1:11000", 2) # two gigs on the other box  );

More information in docs/memcached.txt