Question : two domain name in one domain controller

i have domain controller name
test.com
mail.test.com  (exchange server)

i want to have  secondry domain, eg : students.test.com in same server (tests.com)
can i do this without creating a child domain?,

Answer : two domain name in one domain controller

No, secondary email addresses are not used for authentication.  To do what you are describing you would need to add a UPN suffix to your forest to match the email alias and configure the relevant users to use the alternate UPN in addition to the secondary email address.  Adding UPN suffixes is described here: http://support.microsoft.com/kb/243629