You can not use two partitions at the same time. But what you can do is:
1) Stop all services like postfix, apache and the pop3 daemon:
2) rename /var/www to e.g. /var/www_old
3) remount /mnt/partition2 to /var/www (dont forget to change the mountpoint in /etc/fstab too, so the partition is correctly mounted when you reboot the next time).
4) run mv -f /var/www_old/* /var/www/
5) Start the daemons again.