This is my first post in Ardour mailing list. I want to do the following.
I have two identical audio streaming inputs one is a(t), and other delayed by T seconds a(t + T). I want to switch between these audio streams such that it should be seamless, means the listener should not notice any switching, it should be seamless for the listener.
Is it possible to do real time in Ardour, capture the two audio inputs from sound cards and process and crossfade it and play it out through sound card.
You could do that, although specifically for cross fading just two streams like that a DJ oriented tool like MIXXX might be better suited.
But to your original question, you would connect the two streams to two channels, on the channel which was ahead insert a delay and set the delay until it matched the channel which comes in delayed, then you can control the two channel level independently. I can’t think of a way to make one channel decrease in level automatically when you increase the level of the second channel. Maybe you could get a four-channel panner to do what you want, or maybe a lua script to invert fader moves from one channel and apply to the second channel. I’m not sure whether lua scripting can do that or not, just searching for ideas.
You discussed this on the LAU mailing list. Ardour has really nothing to offer you here, for all the reasons described in the mailing list thread.