Hi
Most folks use this forum for just tech support, so there's not the typical community traffic. Any members with more info...feel free to jump in.
No - you cannot safely "split" or "tap" thermocouple lines. The voltage they carry is extremely small, and the extra impedance would cause the temps to be inaccurate. Since factory thermocouples are utilized in safety functions, you want to avoid that at all costs.
But…that appears to be a dual tip thermocouple. One set of leads should be wired to the roaster electronics. The other set may be already wired to a Phidgets Temperature Sensor (from a previous owner maybe). See if you can trace those wires and find the temperature sensor - either a 1 port or 4 port (pics attached). If so, the temp sensor may be enclosed in a clear acrylic case, with an open port for a USB cable.
If that's the case, Roastmaster can talk to the Phidget Temperature Sensor, but since there is no USB port in iOS, you need a WiFi "host" to send the signals wirelessly. The Phidget SBC3 is a good choice.
If you're unsure, I'd give Diedrich a call and ask for clarification. There's a good chance that one set of leads would be free for data logging - regardless of the hardware installed. So, if there's no Phidgets Temperature Sensor already installed, you may still be able to use that probe without drilling a new one.
Also - you have it backwards - the ThermaQ Blue only supports K types (not J). So, if the second set of leads is free, and the probe is a K type, you could opt for that for direct Bluetooth connection.
Keep me posted, and let me know if you have other questions in the mean time.
Danny