Hi, here is a reproducible version of your data.
df <- tibble::tribble(
~time_s, ~dS2_mV, ~oS2_mV, ~dS2_mr_cs, ~oS2_mr_cs,
0, 211L, 421L, 1.08, -12.7,
0.5, 211L, 421L, 2.16, -25.4,
1, 212L, 421L, 3.47, -37.9,
1.5, 211L, 421L, 4.55, -50.7,
2, 212L, 421L, 5.86, -63.6,
2.5, 212L, 421L, 7.39, -76.5,
3, 212L, 421L, 8.7, -89.5,
3.5, 212L, 421L, 10.2, -102,
4, 212L, 421L, 12, -115,
4.5, 212L, 421L, 13.8, -127
)
Here are some examples, though whether secondary axes are a good thing is up to you:
Creating A Dual-Axis Plot using R and ggplot (finchstudio.io)