Hi. Sorry I haven't responded sooner - the last few days were hectic.
Unfortunately, no - you can't split a thermocouple feed in any fashion. There's a number of reasons, but the biggest is that the voltage is so minute it would almost certainly affect the temp readings. Some folks do it, but I certainly wouldn't advise it–it's not safe.
The best thing would be to install a second thermocouple in the roaster. Depending on the position of the thermocouple, it's often possible to commandeer a screw hole (if it's drilled all the way through) and use for a thermocouple port. Thermocouples can be safely bent, as long as the bend forms an arc instead of a sharp corner. This can help placing the second probe.
If you didn't want to install a second probe, you may be able to find a "dual element" thermocouple, similar in size to the factory one. These are essentially two probes in one, with 2 separate sets of wire leads: one for the PID and one for the Bluetherm Duo.
Give a call to Ambex–this is probably a question they get fairly often. They may be able to point you to a dual element probe fairly easily. If you need any info from me, just let me know. I'd be happy to help.
Danny