sweetbeats
Reel deep thoughts...
Very cool!
An important skill to learn. And none of my cards have new relays mounted. I have new relays on-hand to do this, but the 58 refurb project is a little ways down in the docket just yet.
You'll have to make sure your computer will support the multiple audio streams over the same or different protocols, consider how you are going to handle wordclock master issues and such. There's no quick answer to your "isn't going to cause problems for me is it?" question...
It will also depend possibly on your DAW software. I will wager it is more likely than not you won't have a problem, but it depends on your computer hardware, DAW software, and the individual interfaces you use.
I use a MOTU 8M and think its wonderful.
I also use Harrison Mixbus DAW software which is also wonderful and wouldn't have trouble handling multiple interface devices of the same or differing protocols (i.e. USB, Firewire, Thunderbolt, etc.)
[EDIT]
Rather than two separate interfaces, consider one interface that has a number of hardware analog inputs and outputs, and also some other digital I/O to which you can add your second set of analog inputs/outputs (such as ADAT Lightpipe). That way you have all the I/O you need but just one interface; one audio stream to/from the computer using the one interface device. I have a Presonus Digimax FS I can piggyback on any interface that has ADAT Lightpipe I/O if I need more than the 8 inputs on the 8M.