Exactly. All you have to do is partition the second drive first, then just simply mount it using the following:
mount /dev/sdb1 /home/domain/public_html/hostedfiles
Also, to make it permanent, make sure you update your /etc/fstab file as well.
The info mentioned above is for the case you've described first. If you want to have a general increase in speed, you should really consider creating a mirrored RAID array from your discs (I think it's called RAID 1). You'll only get half the capacity (since everything will be copied to the second disc as well), but it'll be faster and provides some redundancy as well (i.e. if one of the discs crashes, you'll just replace it and that's it). You'd need to backup your data and completely reinstall your system to do this though.