No problem using ADAT to add ADDA channels - or Spdif to add stereo ADDA.  The 2nd interface does the conversion work and shunts the digital tracks over to the interface.  They just need to sync clocks - the 2nd interface doesn't even need to be hooked up to the computer if it remembers settings.
 
@

Hi, Rodreb. You can use two units with ASIO. One unit is the "master" and the other is slaved through the master. I am using the ASIO driver. Any unit to my knowledge that has an ADAT input would work. As was just stated, the second unit does not hook up into the computer directly so ASIO is good with that! Lol. You CANNOT use two separate sound cards running them both into your computer separately using the ASIO driver. If you use the Steinberg card you can even get 24 channels but I haven't tried it myself. Thanks

I use a Saffire Pro 40 connected via ADAT light pipe to my Yamaha O1v mixer which gives me 8 Channels in from the Pro 40 and another 8 Channels in via the O1v which I can actually have 16 analogue inputs to routed to the ADAT if required.
 
Focusrite do a separate eight in that could be connected via ADAT to a Pro 40.

I use the RME Fireface 400 and the UFX. 
 
I've had the 400 since it was first released and it's been rock solid and only feels like it will get phased out if firewire disappears.
 

I use Focusrite Saffire Pro 40 and a Motu 896 as my main units - I have a couple of the Focusrite Saffire Pro 40 on different systems - NEVER had an issue with those - did with the Motu but their customer service was excellent, so I would recommend either.
Nigel
 

I have a Presonus AudioBox1`818VSL audio interface, and I just picked up a Presonus Digimax D8 to connect through ADAT to give me 16 Instrument/XLR inputs.
 
I think the AudioBox 1818VSL ran around $400-$500, and I picked up the Digimax D8 used (great condition) for only $150.
 
If your audio interface has ADAT ports on it, you can pick up an expansion unit (like the one I list here), and there are multiple folks that make 8-port expansions that seem to all work via ADAT, so you should have some choices.
 
Bob Bone

  I'm currently using a Focusrite Pro-40 and I'm happy with it. I've actually connected my old Adat Xt with a Nady 8 channel mic pre to expand to 16 tracks.
  I'm really intrigued by the Presonis Studio 192 that's being released shortly. It has mic pres that are remote controllable through an Ipad app. To anyone that's ever been in their studio alone trying to record yourself playing an acoustic guitar, this is a very cool feature. 
  Your sitting with your guitar comfortably in your chair with the headphones on and the mic in the perfect position and the preamp gain isn't right. Now what. Take off the headphones ,put down the guitar walk to the other room hoping that you don't move the mic, to adjust the preamp gain and try it again.
  The rest of the specs look pretty amazing also. I'm curious about USB3 latency.
  It looks like they've raised the price $100 since it was announced though.It's up to $900.00. I guess I'm not the only one impressed with the specs.
  The add on unit with the DB-25 connectors looks pretty slick too. 

RME Babyface for me.

Hey Joe (see what I did there?) that Presonus unit certainly looks interesting.
 
The spec makes pretty good reading and I'll be in the market for a new interface soon.
 
It's a bit more than what I had allowed for (& sought  permission ) but I reckon I could put on the old Jonesey charm and do a bit of sweet talking.

The RME UFX has 12 analog inputs. If you want to get to 16 (or 20 at 48kHz or less), it's cheap and easy to add a Behringer ADA8200 ADAT.
 
The Behringer is so cheap that you wonder what you're getting into, but it's actually not bad at all. I try to use it on less critical tracks, but I've honestly never heard any problems with it.
 
When my ADA8000 died, I looked for something a little pricier, but it wasn't easy to find another ADAT device with preamps that didn't cost more than my UFX. There appears to be an empty niche in the market.