How you adding output capture then, for two devices?
You should give details on your problem. For example, I'm trying to capture three audio cards (first one model:..., second one model:..., third one model:...) installed in my PC. When I adding 'Audio Input Source' (or 'Audio Output Source') to the scene, and there is only card N (model) available under the Device list (source properties) and I cannot choose card M.