Help replacing fluorescent light fixture - 2 sets of black and 2 sets of white
If you originally have (2) black wires and (2) white wires coming into the existing light fixture ,that had been working before, than these are connected to the new fixture.
The black wire from the new fixture connects to the (2) incoming black wires and the white connects to the (2) white wires.
It might be wise to replace both fluorescent fixtures rather than keep one and replace the other with an LED.