in the beginning (around 1.1.1) it was possible to use: --target i386,i686 this is very useful eg build glibc, openssl etc. on rhel-5 and the same apply to the kernel rpm on rhel-6 (in this case eg: i686,noarch). but it's no longer possible since mock do not install the src.rpm twice which means eg when build glibc which move SOURCE6 from the SOURCES/ dir to the buildroot that the second build will fail. so either remove the possibility to add multiple target or fix it (ie. rerun the mock process from the beginning. anyway it's not epel specific it's apply to the fedora package too.
Ok, I checked for multiple arch values in the option parser code and disallow them. queued for next build.