SAE J1708 is different than J1939. And there are also J1708-bus arbitration which is kind of similar to J1939, multi-masters on the same network, but it is made in software, the J1939 using CAN hardware arbitration. There are some overhead on software to fully implementing the SAE J1708 protocols.
We currently have both SAE J1708 simulator and SAE J1708 interpreter released.
An Au J1708 simulator:
The following is a typical J1708 topology: