Hi!
 
I used to use a Pod Studio UX2 for my interface.  A few months ago, I switched to a Focusrite Scarlett 6i6.  I don't know why I switched to the Focusrite.  I guess I thought for some reason it would be better than the UX2.  It's not really better, but worse in at least one way-Using it with an electric guitar. 
 
I posted a question on this and other music forums about electrical hum and many tell me that the single coil pickup of my cheap Squier Strat is the culprit.  But the weird thing is, is that I didn't have the hum when I used the UX2!  Also, the UX2 came with software called the Pod Farm, which had many preset amp models to choose from.  This is potentially handy for me because I'm not an experienced guitarist and don't have a big array of foot pedals and amps and effects.  I just have the guitar and know some chords and would like to get it into my songs at least in a small way.
 
My question here is, would it be possible to have BOTH the Focusrite and the UX2 on my system at the same time, just being able to switch to the UX2 as the soundcard when I want to plug in the Strat?  Or would doing so cause a conflict with each other?   Also, would the software (drivers/etc) for both be able to co-exist on the same system without conflicts?
 
Thanks for your answers in advance.
 
John B.

Sure, you can have them both hooked up. However, if you're using ASIO drivers you'll only be able to have one of them active at a time. Switching to WDM/KS should allow both to be used concurrently. The main restriction is that if you use both to record the same source at the same time, e.g. two overhead microphones on a drum kit, then the two interfaces will have to be synchronized.

I have POD Farm 2, it is a VST2 and I can access it Sonar.  Don't need any Line 6 device plugged in (I have a POD XT).  The POD Farm 2 effects, such as reverb, delay are some that I use often for guitar.
 
The hum you are getting could be a "ground loop hum" since you don't get it with the Line 6 unit. 
The usual cause of a ground loop hum is a "difference in potential" of two connected AC powered devices.  This is a fairly common problem with amps and effects.
 
Isolating the AC power ground on the Focusrite unit may fix the hum, if it has a 3 prong AC power plug - use an adapter for the AC Power plug that eliminates the ground.  ONLY use it this way when the Focusrite is connected to the PC, as it will get the ground from the PC.   If the Focusrite AC power plug is only a 2 plug (no ground), reversing the plug in the AC power receptacle sometimes fixes it.  And finally it could be an AC power problem in the house wiring. 

Google can be a wonderful thing, I did a little search on "muliptle audio interfaces in sonar" and found a utube video on it, couple of very nice articles from sound on sound magazine explaining the limitations of ASIO drivers, and the work arrounds, plus links to Cakewalk support articles.
 
Basically ASIO standard only supports only one device at a time, unless the devices and drivers are coded in such a way to work together (same manufacture normally), so the work arround is to use wdm drivers or asio4all (which is wdm driver pretending to be an asio driver).  The actual software device drivers, not the asio drivers, should be ok, jsut as its ok to have a mouse and keyboard working at same time, unless you were to have two identical devices installed then their maybe some confusion.
 
https://www.youtube.com/watch?v=8UsSqa4gohQ
 
http://forum.cakewalk.com/Answered-Can-Sonar-X-run-multiple-audio-interfaces-m3034458.aspx
 
http://forum.cakewalk.com/Using-ASIO-with-multiple-audio-interfaces-m1397538.aspx
 
http://www.soundonsound.com/techniques/using-multiple-audio-interfaces-together
 
http://www.soundonsound.com/sound-advice/q-how-can-i-use-multiple-audio-interfaces-together
 
Hope this helps, oh, and as someone with a lot of history in computers, make sure you document any changes you make so if u do go and play with settings, you know how to change them back.

One thing that may work for you is SPDIF.  Use the Focusrite as your main interface through SONAR and use the SPDIF output of the Line 6 into the Focusrite SPDIF input.  SPDIF is all digital so in this scenario you are essentially just replacing the analog circuitry/AD converter on the Focusrite with the one on the Line 6 that you prefer.  
 
I don't know how POD Farm's licensing works but as fireberd suggested, you probably can just use it as a plug-in through SONAR.  If they have licensing on it that requires the device to be available, I suspect it would still work if the device is plugged in via USB even if the device isn't what you are actively using in SONAR.  

for (at least some of the) line6 plugs, the hardware acts as the license dongle. so in my case I have the Pod X3 Pro connected via USB and use the PodFarm plugs, but do not use the Line6 as an audio interface (although I could anytime just by unchecking the other audio interface and checking the Line6 IN/OUT in Sonar preferences)
 
I have the Line6 connected via SPDIF to the other interface i.e. there is no additional DA - AD conversion adding latency. However, you need to check the manuals of both interfaces to get the clocks syncronised. otherwise you will end up with clicks and pops or just drifting recordings that won't line up and have phase issues.
 
The hum is probably a different issue in itself, caused by some sort of ground loop or "dirty" voltage from the USB. this can be everything from easy to very difficult/impossible to resolve depending on your devices and power supply. you may have to read up on groundloops and how to avoid them ... just don't fall into the trap of buying USB cables for a few hundred bucks. they won't cure it ...

Thanks all for your tips.  I would use the Focusrite for everything except the electric guitar.  For that I'd switch to the UX2.
 
I find it odd that the Focusrite uses an AC adapter to plug into the wall AC as opposed to the UX2, which gets its power from the PC.  I was hoping that meant better memory preservation, but it doesn't.  In fact, the Focusrite seems to be even more of a memory hog than the UX2.
 
John B.