How does SAT subject test work?

Hey so,i got 600 on bio,(ya i know its awful) and 660 on math level 2. So i,am considering retaking bio since I didn’t really study for it back then,and i personally think getting a 600 on subject test while having an A in ap bio does not make sense at all. Anyways so I am debating whether I should retake Bio m with US history or take it with Math Level 1 (because i didnt do that well on math level 2 either) Which one of those will be an “easy” choice for me and do you guys think that its a good idea to retake bio subject test?
Is it possible for me to retake bio subject test then send that score with previous math level 2 score?