Check first that both cameras individually (depending on what they are looking at) can run at 30fps by ensuring that the exposure is not see too long. Its very common for certain types of cameras to slow the fps due to lower light. This can typically be avoided by unselecting the auto-adjust for the exposure. Yes, you do get darker images, but sometimes that's better than getting fewer images.
1. Using the Camera Properties module you should see both cameras listed as something #1 and something #2. This is RR trying to allow you to select the right camera. It will use the serial number from the camera to keep this numbering the same on restart ... but there is NO guarantee this will work as many cameras do NOT have serial numbers. I think most of the Logitech cameras do so you should be ok. If this is not the case, you can try running one of the cameras through a USB hub which can sometimes cause the OS to assign it always as the second camera. The OS doesn't have any other way of identifying which camera is which so if neither of these work, it will randomly assign which one is first and which one is second in the list of cameras. Not much we can do from there ...
I think some folks in the past have added a known calibration process to help tell which camera is which and automatically switch if needed. Ie. if on startup one camera is pointing at a large green object and #2 at a large red object, you can determine which is which visually. Its a hack for sure, but when you don't have any other options.
Others will avoid using the same camera model. Different model, no issues on which is which. This issue only happens on two cameras of the same make, model.
2. Experiment with the different camera "Video format" seen in the Camera Properties module. RGB32 is the largest and requires the most USB traffic. MJPG (if available) will be the smallest format but will also corrupt a lot of the color information. YUV etc fall inbetween those two and can provide a similar image for less USB bandwidth. Exceeding the USB bandwidth will reduce the fps, so using a more compact format will allow more cameras to run on the same bus.
3. Sometimes plugging each camera into opposite sites of the PC will use two different USB buses ... typical on laptops but not on tablets. Not sure on your setup but sometimes that can help to distribute the usb load.
Hope some of these help.