getting caches to consider each other siblings

Hello Squidders,
 I have two caches running as accelerators. I wish to turn them into
siblings, in proxy-only mode; my understanding is that this should
effectively double the size of my cache at the cost of some overhead on
cache misses.

Calling the caches "humpty" and "dumpty" for the moment, I have added
the following:

cache_peer dumpty sibling 80 3130 proxy-only no-query

to humpty's squid.conf, and done the equivalent to dumpty. And, of
course, I have restarted the squids. However, the access logs show no
evidence that the caches are asking each other for objects; they seem to
be going straight to the source. Can anyone please advise me as to what
I may have missed? Thanks in advance.

--
 Ben Rosengart
UNIX Systems Engineer, Skunk Group
StarMedia Network, Inc.