Duncan,
Thank you for the advice.
I created a custom template in
/etc/e-smith/templates-custom/var/qmail/control/smtproutes/
and called it 30CustomRoutes. That template has only one line in the format
domain.name:[ip.address.of.the.second.server]
Square brackets are neccessary if you do an IP address instead of a name.
Than I just expanded template
/sbin/e-smith/expand-template /var/qmail/control/smtproutes
and everything works.