My suspicions are confirmed according to the diesel guys also confirmed with Aeroforce:
The problem you're going to run into is that you cannot have two OBD "scan" devices on the CAN bus at the same time. At least not both running at the same time. The reason (and this may or may not make sense) is that scan tools/devices use a specific node ID to request data from the PCM. If you have two (or more) devices on the bus and both are requesting data, the PCM will handle each data request in the order it was received. The problem occurs when the data is sent back to the scan device. Since both devices are listening for a response, both may receive the data but only one will be the right device. This may cause one (or more) devices to display incorrect data and may even cause one (or more) devices to freeze up.
The only situation where this may not be an issues is if you are using one device to monitor transmission only data and another device to monitor engine only data, and ONLY in the case where the vehicle has a separate ECM and TCM... like the 6.0L, 6.4L, 6.7L, etc. Since the ECM and TCM use a different CAN Node ID (0x7E0 for ECM and 0x7E1 for TCM), there is not going to be any conflict as to which device gets the intended data.
There are some other issues relating to bus termination that may also be a problem from running multiple devices on the same bus, but that's another story.
All of this boils down to the fact that it's not really advisable to run to more than one OBD device at a time. If you do, expect unexpected results.