To create a second primary disk, you'll need to use a partitioning tool to hide the first primary disk. After the second installation, you can then unhide the first one.
Rather than play this game of hiding and unhiding partitions, it's best to use a boot manager
to take care of the whole thing. Some such products are :
BootIt NG ($34.95)
Support of over 200 primary partitions (if desired). I've used it and it's probably the best.
GAG (initials, in spanish, of Graphical Boot Manager) (open-source)
Allows boot of up to 9 different operating systems installed in primary and extended partitions. Never used it.
Partition Commander 11 ($49.95)
No info on maximum partitions number. I've used it a long time ago and it worked then pretty well.
Norton PartitionMagic 8.0 ($69.95)
Otherwise called BootMagic, no info on maximum partitions number, and seems to only support partitions up to 300GB. Never used it, and I usually stay away from Norton products.
Warning: Before playing with partitions, take great care with your backups.