You can effectively daisy chain interfaces to have more connectivity with more interfaces but it does not increase your overall channel count. It also greatly complicates routing. If you're doing mixdowns on daisy chained interfaces you can essentially get more out of the channels you have.

But if you actually want more channels you can run multiple (up to 3) RME interfaces with the same driver type via one PC. So if you ran 2 MADI XTs you'd effectively have 788 channels, of which all can be simultaneously exposed to an application with ASIO support. It's essentially seen as a single interface in that regard. However, you'd need to clock the interfaces via their bnc word clock connection, or via an external word clock if other attached interfaces can't clock over MADI. You also have to control each interface with its own instance of TotalMix. Not that that'd be too hard, just something to note.

Personally I'd think you'd want to run at least one MADI "backhaul" lane between the two MADI XTs, so to speak. This would allow you to share channels between the two interfaces if the need arises, and you can sync clock over said connection. This would give you another MADI connection to play with, and headroom to add another MADI XT if needed. If you have no need for routing channels between the two interfaces, then you can simply use a dedicated word clock connection. Which would open up all 3 MADI connections on the second MADI XT.

Lastly, other interfaces use the MADIface driver. Like the UFX III, for example. So you don't have to get another MADI XT if you want something with fewer channels.

No it will automatically be seen as an aggregated device in Ableton.

I could be wrong but I don't believe these use the MADIface driver. Even if it did you couldn't use 4 devices at once, you can only use 3 of the same driver type at the same time.

I've been considering this as well. But it shouldn't really affect anything as long as you're routing it into one of your MADI XTs. If I were in your position I would be using 2-3 MADI XTs and make sure that everything interfaces with at least one of them as opposed to trying to mix and match dissimilar cards with the same driver.

Interesting... I didn't realize there was a difference here between operating systems.

Just to clarify - you're saying there's an additional step or setting you have to enable to see two interfaces in one application simultaneously? Or are you saying that channels must be literally aggregated on a single interface?