I currently have a firewire interface. I am thinking about getting a new ART digital preamp for vocals, and it has a usb output on it. I have in the past used my interface and a usb midi device without any issues. Do you think it would work to use both an usb audio input and the firewire interface at the same time?  I know this is a really general question, however I do not recall hearing about using multiple device inputs simultaneously.
 
Thanks!

The ART is not a sound card/interface. Use Art as a pre-amp and your sound card as a sound card.
ART ---> Sound card's line input or pre-amp.
 
You cannot use 2 different sound cards for recording. There will be massive clocking issues. In some instances, you can use one for recording and one for playback, but you cannot use ASIO driver modes. Its always best to use one sound card for recording and playback, unless you want to upgrade your signal chain and spend 4K and up on DAC's, one for A/D conversion and one for D/A conversion. 

The short answer is that these 2 devices are doing 2 different things, and should not interfere with each other.  As noted above, only 1 of the 2 devices is an audio interface, and the other is simply a pre-amp.  Further, they are each using a different connection protocol (Fire Wire and USB), which also will not trip up on each other.
 
ASIO supports a single driver running, but within that limitation some interfaces DO allow multiple interfaces of the same brand/model to be present - so that if you have an 8-input device you may be able to add a second 8-input expansion interface.  This is certainly not universal, and certainly would not work in ASIO driver mode with interfaces from different vendors, to the best of my knowledge.
 
Many folks generalize the above into saying you can't run more than a single interface in ASIO, but as explained above, the issue is with ASIO, and some vendors do support chaining multiple ASIO interfaces.  There WILL be clocking settings to make sure are correct, should you ever choose to run 2 supported chained interfaces in ASIO driver mode.
 
Different audio interface makes/models can run together, but not in ASIO mode - WDM allows them to run concurrently, but may have other issues, like latency-induced dropouts and such.
 
So, you should be fine, and all the stuff in the middle above applies to multiple audio interface, which is not your situation.
 
Enjoy, 
 
Bob Bone
 

Thanks guys!

I often run multiple interfaces with WDM, but not with ASIO.