They're available for sale. We sell a really fancy
OBD-II breakout/splitter box. It's not cheap but it has banana jacks so you can probe every signal on the connector with your multimeter or oscilloscpe. I've seen other places sell less-expensive ones that are a simple cable.
But I would caution you that each device has a network address to communicate, including the scantool. You can't just plug in two tools like you describe and expect them to work perfectly -- they will conflict if they use the same address. Usually when people use a splitters they're "sniffing" the network to reverse engineer the network messages. In that situation they're passively monitoring the traffic and there would be no address conflict.