Is SAT biology E and M merged?

I have found a lot of sites that make these two subjects sound like completely different tests.

On my admissions ticket it says “Biology EM”

Is that both of those two tests merged? I am assuming this has been a recent change.

Do I choose which one to take on test day?

If they are split which one should I take? I got a 5 on AP Biology, and 4 on AP Environmental Science.

Nope; 2 different tests, although they share about 75% of the questions.
https://collegereadiness.collegeboard.org/sat-subject-tests/subjects/science/biology-em