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, the individual wire is connected to the lampholder of lamp 2 on the left side, and the common wire on the right side.
On ballast 2, the individual wires are connected to the lampholders of lamps 1 and 3 on the right side, and the common wires to lamps 1 and 3 on the left.
Using one 1 lamp and one 2 lamp parallel ballast.
The 1 lamp ballast usually connects to the middle lamp, the 2 lamp ballast usually connects to the outer two lamps.
Parallel Ballast Lampholder Wiring