I've never tried it, just asking if it's possible.
 
I have a Focusrite Saffire Pro 40 and an MAudio Fastrack Ultra 8R, each with 8 channel (audio input) capability.  There are times I could use more than 8 channels at a time during recording.  Is there a way to use both at the same time?  Looking at the audio setup, it looks like only one unit at a time can be selected, both in X1 and 8.5.3.
 

It depends on your driver type. If set to ASIO you can only use one interface at a time. However, if you switch your driver type to WDM then you should be able to use two (or more) audio interfaces simultaneously.

ASIO4ALL seems to work.

Hah! Forgot about that. I read recently that ASIO4ALL lets you configure two interfaces as one single ASIO device. That's a good option if you want to keep ASIO performance

I do know that 2 M audio devices work great together. That's what I have had for the last year. 2 AP2496's synced with digitali\o (spdif) So I have 4 in and 4 out and midi in and out on both. Workds beautifully.
I am using WDM drivers.

I also have 3 delta 1010lt in my pc and they communicate perfectly!

asio4all can do that?  that changes everything!
sweeeeeeet!

wdm has always done it, as was stated above

The Saffire Pro 40 won't work in WDM but the MAudio will.  Will that work with one in ASIO and the other in WDM? 

I tried ASIO4ALL one time and it wiped out the Saffire Pro 40 so I won't use it again.

Does it make any difference using two interfaces if one is USB and the other Firewire??
 
I have a Sapphire Pro40 - Firewire, and a BehringerFCA1616 which can be either. I was thinking of using the Sapphire for the inputs and the Behringer for the outputs.
 
Anyone see a problem with that arrangement??