Can I apply to two different GSAS degree programs at the same time?

Yes, but two separate applications are required, each with its own supporting letters of recommendation, transcript, GRE scores, etc. Each department will then independently consider the application. If admission is granted to both programs, the applicant will need to decide in which program to enroll. Once admitted, note that students may cross-register in courses offered by the other program, so there is flexibility to design a program that suits his/her needs and interests.