Is it possible for a student to sit for more than one GRE subject test in one sitting? For instance, back in high school, I was able to sit for multiple SAT subject tests in one sitting. I, along with other students, had the option of sitting 1, 2 or 3 tests of our choice. Is there a similar policy for the GRE subject tests?
No, you may only take one test during a single session. Unlike the SAT subject tests, you get three hours to do one test, not the other way around.
Source: I've took a GRE subject test last year.
Generally, these tests are set at the same time. So it's impossible to take two tests at the same time.
I couldn't find an actual statement that only one subject test may be taken on a test day. However, in the FAQs, there is this question and answer:
How much time should I plan to be at the test center?
Plan to be at the test center up to three-and-a-half hours. Report to the test center no later than 8:30 a.m. If you arrive late, you might not be admitted and your test fee will be forfeited.
That implies that only one subject exam can be taken on a test day.
There is only one testing time per day, so you can't take more than one test in a day (ok, if you flew east across the international date line, you might be able to pull it off).
Theoretically, there are multiple testing times in the same week. The GRE does offer Monday testing in addition to Saturday testing, though this is supposed to be only for religious and cultural purposes. (I say 'supposed to be' because somehow my own subject test was scheduled for a Monday, which I realized after not being admitted to the testing center on Saturday.)