Fluorescent tube light fixtures with two ballasts are typically wired in the same manner. Individual wires of one ballast connect to the one side of the fixture, individual wires from the other ballast connect to the other side of the fixture. The common wires of each ballast are connected to the other side of the fixture from each of the individual wires.
As seen below on ballast 1, individual wires are connected to the lampholders of lamp 2 on the left side, and the common wires on the right.
On ballast 2, common wires are connected to the lampholders of lamps 1 and 3 on the left side, and the individual wires on the right.
Using one 1 lamp and one 2 lamp series ballast
The 1 lamp ballast usually connects to the middle lamp, the 2 lamp ballast usually connects to the outer two lamps. If one of the outer lamps fail, the other outer lamp will not light up.
Using two 2 lamp series ballasts
Series Ballast Lampholder Wiring 3 Lamps