yes, you can do that...
The BIOS will boot up witchever Operating system is on the First DIsk in the Boot order in the Bios's setup options.
you can change witch disk you wont to be first in the boot order threw the BIOS.
Iv never Installed 2 OS's on one system b4... but have experince with the Boot.ini with for example the Recovery console and whatnot.
and if you wonted.. i believe you can edit the "boot.ini" on the primary Windows install, to list the other OS on the other partition, so you can manually select witch OS to use without going into BIOS. (this may not work for linux - like i said i never installed that).
you can edit the "Boot.ini" from... /control pannel/system/advanced/startup & recovery/settings/sys startup/edit
you can also edit the amount of time you wont to display the List of Operating systems at Bootup, from that same tab..... witch ever OS is listed first in the Boot.ini is the Default... so if you set the time to display the boot list at "0", that OS will automatially boot.
EDIT: the only reason the Boot.ini doesnt automactially contain the new OS, is because your installing it on a seperate /disk or /partition, so it doesnt affect the Primiary OS at all... but you can just copy and paist the OS description from your Secondary OS.. into the Primiary Windows Boot.ini
you can manually open the Boot.ini file by going to /start/run, type (c:\boot.ini) but you wont beable to edit it from there, only copy. If you install another WIndows, it will be in the same location, just change the Drive Letter.