UPN has picked up not one but two erstwhile WB Network dramas for its fall 2001 lineup. UPN clinched a deal with 20th Century Fox Television and Regency Television on Wednesday for "Roswell," which was dropped by the WB earlier this week after two seasons. "Roswell" will be paired in the fall on Tuesdays with "Buffy the Vampire Slayer," which UPN acquired by outbidding the WB for renewal rights last month. Next to "Buffy," the new installment of the "Star Trek" franchise, "Star Trek: Enterprise," will be the highlight of UPN's fall schedule presentation to advertisers in New York today.