[exim] Multiple AV scanners

I'm currently deploying a standby content scanning server. I know that
more than 1 SPAMD server can be specified in the Exim config and my
understanding is that if 1 is down it will try the next, much like
specifying more than 1 SQL server, etc. What I'm fuzzy about is the
av_scanner property, I want to follow this same scheme with clamd.

I would think this should work:
av_scanner = clamd:host1 3310 : host2 3310

If I deploy my config with the above option, when I shut down clamd on
host1 I get a failure and the message bypasses the malware check rather
than checking the second server. Note I have defer_ok on my malware
check for the chance my clamd box is down.

Am I missing something here? Doesn't the av_scanner config option
support multiple hosts? It would make sense that it does in order to
spread load across multiple servers. I could use round robin DNS and
point a DNS entry at both servers, but that still does not solve my
problem of if 1 host is down.

Thanks,
James