I am slightly confused by your question. You tell, that you are using an UCX now.
Its a more modern design compared to the outdated Multiface II.

Where do you see a limitation in your current UCX, so that you ask now whether ADI-2 + Multiface II would be better ?

Better in what regards and for what purpose ? To connect monitors or headphones ?

Maybe you should also describe your surrounding environment a little bit better (headphones, monitors) and whats the reason for your question. I assume "number of analog I/O" is most likely not the key reason for your question as the ADI-2 only delivers 2 Analog I/O more (plus headphones).

If you really want or need quality in terms of latest converter technology, then you should perhaps get a recording interface which has been designed last recently, instead spending money into older technology like Multiface II and ADI-2.

If you want to get something really better which also gives you more options then I would look at the UFX II and sell the rest. UFX II will give you a lot more as this is the new flagship interface UFX+ only without MADI/USB3/Thunderbolt. The UFX+/UFX II are according to RME advanced as they designed the analog section and headphone preamps newly. With Durec you would get more options for recording (for more safety or standalone recording). More advanced standalone capabilities by display and menue additions. Better and more Mic preamps which can be used on top of the analog channels and also be switched for instrument inputs. More ADAT channels, so that you can expand i.e. with more preamps and at the same time use ADAT2 for optical SPDIF to i.e. connect to HiFi to make your Recording PC to a high quality player or to use your HiFi as 2nd or 3rd monitor.

If you want even more quality and features (the ADI-2 Pro and ADI-2 DAC offer a lot of features along with converter quality) then get additionally the ADI-2 Pro or ADI-2 DAC depending on what suits your demand better.