Hi, Can you tell us more about your webcams and your setup ? Windows ? mac ? linux ? your OBS Version ? what are your webcams ? what are they capable to do (Video Resolution for example).
In a first time, are your webcams working normally one by one, with the regular software (ie logitech ? microsoft, etc...)
In a second time, we can check the sources. Can you tell us, the way you've add the sources (precisely, every options you've checked, chosen, selected... maybe a screenshot would be nice too).
When you've add the sources, do the image fill the preview screen ? are you sure the first cam is not just "above" the second one (and therefore "hide" it) ? (it can be checked by changing the order of the sources, though i don't think this could be a problem since you've said you've had one cam per scene in a first test).
Regards