It has been a long time ago since I set my Ur824’s…
I do recall you need to start by plugging in the USB cable from only the slave unit. Open the dspMixFX UR824 control panel and set that unit to slave.
Then unplug it’s USB cable and plug in the master UR824. I believe then the dspMixFX control panel will show the slave channels. I think it just works then.
There was something about setting the master clock to internal on the main unit, and ADAT on the slave one also if I remember correctly.
I recall I found this information from Sweetwater Support. Let me know if this works for you. 