I have two tracks: one is 15 sec. and another is 25 seconds long.
I want to select 15 seconds of the first one and 25 seconds of the second one. Is that possible? I have read many Audacity tutorials but no luck so far.
All I can do is select the same length on both tracks (pic 1)
No, Audacity does not currently do that. If a selection is across multiple tracks then it is the same time period in all selected tracks.
What are you wanting to do? Perhaps there is some other way to accomplish the task.
because right now if it is applied to multiple files with different length, it applies fade in correctly to all files but it only applies fade out effect to the longest one(s) (see pic).
So it thought to solve it by selecting only active part of all tracks.
If there are only a few tracks to do, select one track and apply the effect, then select the next track and press Ctrl+R to Repeat the effect. You can select a track by double clicking on the waveform.