My guess is that this is an issue with the certificate in use which is definitely going to be different between the servers. We are working on making this smoother but in the meantime you may need to use a statescript with some extra logic on your end to update the server.crt file.
I think if you deploy an update that adds the second ServerURL but does not update /etc/mender/server.crt, then the deployment should complete from server1. You can then use a ArtifactCommit_Leave state script to update /etc/mender/server.crt (or just delete it and remove the conf file entry if your new server uses a CA-signed cert). At this point you may need to restart Mender.
Please let us know if you try this method and what success you have with it.
Drew