[exim] Backup virus scanner

    Would it be possible in exim 4.5 to specify two different
antivirus scanners in a way, that second scanner is only executed,
when first reports an error (clamdscan and clamscan for exampe - if
clamd has crashed, then clamdscan reports error, clamscan is executed,
if clamdscan does not report error, then clamscan is not executed). 
    I'm currently using Debian Woody with Exim 3.35 as mail server and
I use modified old perl exiscan for virus scanning (modified to
support backup scanner). But now, with the Sarge released I'd like to
upgrade my Debian box to Sarge and to start using exim 4 - rejecting
viruses at SMTP time seems like a nice feature.