Addendum: As the Line In of Mono Input 2 is of balanced nature, you want to connect the (most likely) unbalanced output of the the Skype soundcard via a DI-box. This may be a rather simple passive one, but make sure that it is capable of transforming the audio signal 1:1, i.e. without matching the level to a microphone input – also known as line isolator. (After all, you are using the Line In for Skype, and not the Mic In.) And, importantly, use the correct audio cable, particularly if you are dealing with a 3,5 mm- (¼"-) stereo jack on your sound device. The exact model depends on the type of DI-box you are utilizing, but make sure that in case of mono inputs (which is what these boxes ususally have) you are applying a splitter cable, of which you are using only one end on the DI-box-side.
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TSD
Edit: Typoes