I assume you are going for the 2 ohm load, correct? There are two portions of the diagram that require a single conductor wire: just the parts where you connect the + of one voice coil to the - of the other on the same subwoofer. All of the other parts of the diagram can be wired with speaker wire with 2 conductors. When you wire things up, you don't have to have the wires in the exact positions that they are in the diagrams, it only matters where they end up. They draw them out like that so you can trace the path. It makes it easier to understand what is going on.
You can do that wiring job with the following:
-2 lengths of wire with 2 conductors
-2 short lengths of wire of single conductors
The first length of wire goes from the speaker terminals of the amp to the + of one voice coil of subwoofer A and the - of the other voice coil of subwoofer A
Now connect the remaining + on subwoofer A to the remaining - on subwoofer A
Now use the second length of wire. Connect one end of this wire to the exact same terminals that you connected one of the ends of the first wire. For example, you can connect the first end of this wire to either the amp speaker terminals, or you can connect it to the terminals of subwoofer A that you connected the second end of the first wire to. Make any sense?
The other end of this wire gets connected to the + of one of the voice coils of subwoofer B and the - of the other voice coil of subwoofer B
Now use the remaining small wire to connect the two remaining voice coil terminals of subwoofer B together.
I hope I made some sense instead of confusing you more. It would be so easy if I could just show you. Maybe there's a video somewhere...