I found the answer in this obscure posting at: https://obsproject.com/forum/threads/using-2-or-more-logitech-c920s-or-c930es-together-successfully.26707/
I've edited in what I learned by doing it. It works!
"I was having problems setting and retaining my settings while using multiple Logitech webcams with OBS. I have 2 Logitech c920's and 1 Logitech c930e (so far), and i could never get them all to work properly together at the same time so i combed the nets for a solution on multiple occasions. The other day i found a solution in this forum that works for me and i think will work for others: http://forums.logitech.com/t5/Webcams/Can-t-configure-multiple-C920-webcams-on-single-PC/td-p/950226
I will explain as well as i can, please don't expect any follow-ups if it does not work for you... Although i may check back from time to time. A user on that forum named MarkJeronimus wrote a program on the Java platform called AdvancedCameraSettings (ACS) to allow you to adjust and save the settings from two logitech cameras in a standalone pop-up interface. There are a few steps to installing and using it-
0>Uninstall the Logitech default software, BUT LEAVE THE CAMERA DRIVERS INSTALLED.
1>Install the Java Developers Kit: http://www.oracle.com/technetwork/java/javase/downloads/jdk8-downloads-2133151.html Java is a programming environment, Google it now if you are not familiar with it. I am on Windows 7pro and i installed the 32bit version [x86] of 8u40 even though i'm on a 64bit system, i don't think it matters though. [I used the latest Java 8u51 without problem.]
2>Download MarkJeronimus' awesome self-contained program: https://dl.dropboxusercontent.com/u/13260068/AdvancedCameraSettings_r2.zip Unzip this download and put the resulting folder in a reliable location. [I noticed settings changing when i moved the folder after making and saving adjustments, so put it somewhere it can stay & place a shortcut on your desktop.]
3>Plug in both of your webcams
4>Launch the Java program AdvancedCameraSettings_r2 On my first launch, the program seemed to crash Java and gave me an error message.. but i found that unplugging one of the cameras and launching the program worked, then i closed ACS again and plugged the 2nd camera back in and reopened ACS. Small hassle for me, but totally worth it anyway.
5>Choose camera 1 The program shows you a list of attached cameras, I just chose my cameras from top to bottom, first camera listed is camera 1 for me. After you select the camera a list of codec's appears. On my c920's i get to choose from: RGB24 or I420 or MJPG or H264 On my c930e i get to choose from: YUY2 or MJPG I am unclear on what EXACTLY the program is asking for.. My natural choice would be h264, but MJPG is the only one common to the c920 and c930e and it seems to work well, so that is what i use: Video, MJPG, width: 1920, height: 1080, bitDepth: 24, fps: 30.000029 If anyone understands this section more fully, any comments would be appreciated.
6>Choose camera 2 Same drill as camera 1.. for me camera 2 is the 2nd camera on the list.
7>The interface will open and present LOTS of options for your camera, including a Load and Save function on the last tab. If you launch AdvancedCameraSettings before you launch OBS, the camera's output will appear in ACS's interface. What i do is launch OBS first and create a scene with both cameras in it and then launch ACS and just Load my favorite settings.
That's about it... DONE. It may seem complicated at first, but if you have 2 c920's you probably understand. Really, i might use this to adjust settings even if i only had 1 camera because of the expanded options. I would recommend doing this when your computer is not likely to be changed around alot and pay attention to what usb socket each camera is plugged into.. Once you get something you really like and your set is not likely to change much you should screenshot all of your settings just in case.
-ADVANCED-
Okay, if you got all that working and STILL NEED MORE, i found that you can run more than one instance of MarkJeronimus' AdvancedWCameraSettings and control more camera(s) with a 2nd ACS interface. I mentioned earlier that you should put the folder containing the ACS program in a reliable place, now simply duplicate that folder and rename it accordingly. I renamed the actual program files accordingly, my first instance is for the c920's and i labeled it "c920 ACS" and dragged a shortcut to the desktop. My second instance, in the duplicated folder is for the single c930e and i labeled the program "c930e ACS" and dragged a shortcut to the desktop. Each folder has its own configuration file and the Loading and Saving function seems to work great so far.
THANK YOU MarkJeronimus!! There are problems that persist using ACS that were also a problem with the original Logitech software (such as Gain levels jumping for no reason), but overall this little program completely destroys Logitech's default software. These Logitech cameras are really nice, but i was ready to give up and look for another model until i found ACS. I really appreciate MarkJeronimus for writing and sharing it in that forum. Really, really appreciate it.
I am a complete novice with Java and the like so it took me a couple hours to figure it out, but with this list i think a novice could have this up and running in 20 minutes or so. I think the steps provided should work if you are running a similar system to mine... if not i really have no answers for you. Obviously, you need a computer that can process these CPU intensive camera's, so be aware. Worked for me."