Why buy 2 interfaces when you only need one and then combine that with an ADAT extension like f.e. the Behringer ADA8200.
I use that with the UR816C and have 16 inputs and outputs.
I guess the 824 has no further driver support in the near future? I’m not sure. The 816 is the newest so probably the longest support for now.
The URC is the newest and supports USB 3 (the 824 is USB2).
Also has MIDI ins and outs, the 824 does not have this feature.
It can function within the controlroom of Cubase also but you need the DSPfx mixer to make the adjustments, that cannot be done in Cubase when you use the Controlroom. I don’t have a problem with that, I use the Controlroom.
And yes, they always advertise with the extended connections possible, so the 824 has 2 ADAT connections which gives a total of 24 inputs.
The UR816C only can handle 1 ADAT extender so that makes a 16 inputs in total.