I have never done this personally, but seen it referenced several times. AFAIK, ASIO aggregation is achievable, but seems only possible with units of the same brand which support this (so drivers truly match). I recall seeing a Focusrite Saffire video when I first got mine and the guy jumped into doing this right off the bat.
 
Definitely something to research as you must know beforehand that ASIO aggregation exists, and that the units are capable of talking properly to each other.
 
Just found a post by Focusrite on another board, and this brings up another point that bandwidth can get exceeded by aggregating two large units.

Modded ADA8000 ==========Terratec EWS88D
 
Like my ascii lightpipe?

Pretty sure you can add eight channels with an ADAT (Lightpipe) connection. The ASIO driver sees only the interface itself, with eight additional inputs beyond those that are physically present. ADAT can only accommodate 44.1 or 48 at 24 bit, so if you use higher setting this might not be for you. This article is five years old, but I think the info is still valid. I have never done this myself, so be careful before you spend your dough.
 
[Edited for typo]

I've been using Lynx Aurora 16 with Sonar X1, X3 and now Platinum.  I've been using it for about three years and have been pretty happy with it.  16 channels AD and DA.  I would highly recommend listening to some different convertors.  I was surprised at how much difference it can make.  To my ear the Aurora is slightly bright but not overly so and seems very detailed.  By comparison, my old Apogee sounds good but a little darker.  The Mackies that I have in my hard disk recorder are not that great.
 
I use the Lynx AES16e card.. works fine, but the software interface is bad enough that I wish I had bought the RME.
 
Windows 7 64-bit

Still using my trusty RME Fireface 800
 
I was already a big fan of this unit, but when they rewrote their TotalMix software this year to work on the FF800 - a 10 y.o. or so product - they really locked up my loyalty with that level of support.  That was big.

Hi. I just switched my old Echo AF8 (it was dying) for a RME ucx, and could not be happier.
You would probably need the bigger version. But it was quite a big difference in both latency, ease to set up and may I even say sound.
All the best.

Hi,
I´m using the Focusrite Saffire Pro 40 and a
Octopre via Firewire card (TI chipset ) since years 
without any problems. This packkage "sounds" really good.
 
If you have more bucks for disposal I recommend RME, UAD aso
interfaces cause of better latencies - which is no problem with the saffire range-
but it´s simply easier to handle in that units......
 
Basicely it s a question of how long you want to work with the new interface-
caus firewire/thunderbolt interfaces could be a dead end in the next years for windows based system.
 
 
 

In the past I was using RME 2 x FF800 s + 2 x 8 via ADAT. Without problem.

ADAT passes the audio through the main unit so the ASIO driver sees all the channels including the 8 via ADAT. Some units like the ff800 mentioned at the beginning can acces 2 ADAT in and out giving you an additional 16 channels to whatever the base unit can carry.

In the case of the FF800 the driver allows you to aggregate the 2 devices so the driver just sees it all as device 1 and 2 with each still having access to 2 x 8 channels of ADAT.

SF_Green
Still using my trusty RME Fireface 800

Same here.

RME are great interfaces and their driver support is second to none. They still release new drivers for their very early models which is very rare indeed.