I would install Vista on the smaller of the two drives.
Then insert the Windows 7 ISO and perform a
\"clean\" install to the other hard drive.
If you don't get the boot menu after installing, you'll need to get into the BIOS and change the second hard drive listed to the top position so it can boot from it.
If you're an advanced user you can download Gparted and delete all your partitions (after saving all the information you need to a flash drive and create a partition of about 50GB for the Vista installation and 70 GB for the Windows 7 install. If you do not create any more partition in GParted, just go to Start > Run, and type diskmgmt.msc and you can format your unallocated space and new drive letters will be created.
The reason I use GParted is because you can't shrink the size of the partition any more than half the drives capacity.
You can download GParted here:
GParted -- Download
You need to burn that ISO also.