Speech / Audio Separation
I can advice this ones :
http://www.cse.ohio-state.edu/pnl/software.html
There are papers and codes.

Speech seperation is not an easy issue I advice you read some books before. 
This book which you can check its examples in the book is helping me much: 
http://auditoryneuroscience.com/

The thing you said when the signal start can be one feature ,but Speech sounds change
constantly in a manner that is fast, but not too fast, and somewhat unpredictable yet
not entirely irregular. What I mean you will get false negatives with only this feature.

There are other things like periodicity, amplitude, pitch etc... 
I think trying draw a spectrogram of speech and examining it is the best start.