There is no automatic way to do this. Probably the easiest thing to do would be to create a new mount point in fstab and mount it to /home2 or something similar. Then after you create a new user, home on the command prompt as root and do something like:
mkdir /home2/username
chown -R username:username /home2/username
cp -a /home/username/public_html/* /home2/username/
rm -rf /home/username/public_html
ln -s /home/username/public_html /home2/username
I'd create a test account and try it before doing it with a real client/site first but you might also be able to migrate existing web directories to the second drive with that too.