started feeling bad monday. took a pcr test tuesday morning and it came back negative. had another pcr this morning that was previously scheduled already, and it came back positive.

aren't false positives more prevalent than false negatives? do i just assume i have it? do i need a tie breaker?

I would assume you have if it the second, later test was positive.

Quick Google search shows this from Harvard:

"What about accuracy? False negatives that is, a test that says you don't have the virus when you actually do have the virus may occur. The reported rate of false negatives is as low as 2% and as high as 37%. The false positive rate that is, how often the test says you have the virus when you actually do not should be close to zero. Most false-positive results are thought to be due to lab contamination or other problems with how the lab has performed the test, not limitations of the test itself."

Association of American Medical Colleges
That's because even highly sensitive PCR tests can come back negative if you're tested right after exposure, before the virus has built up to detectable levels, according to a study co-authored by Lauren Kucirka, MD, PhD, an epidemiologist and OB-GYN resident at Johns Hopkins Hospital. "Say you had lunch with a friend who was positive. Day one is the day after you had that lunch," Kucirka says. "What we found was that if you are tested in the days immediately after exposure, the false negative rate is anywhere from 50% to 100%." So just because you've tested negative doesn't mean you're not harboring the virus. The average onset of symptoms is five days post-exposure, and your peak infectiousness is two days before and one day after symptom onset, according to a study published in Nature Medicine.