Multiple Pattern Locks at the same point of time Is it possible to have multiple pattern lock implemented in an android device to increase it's security. I know that it must be possible to design an app to do that, but my question is whether any app has already done it?

Asking a Similar Question, is it possible to install more than one app that lock my screen with pattern lock and use them simultaneously? Will it cause any kind of clashes or other troubles?
To answer the first question - That singular pattern where at minimum four dots must be joined, is built into Android, if you want to have multiple patterns, that requires modding the source and rolling your own ROM which is outside the scope of this site.

Now, onto the second question "is it possible to install more than one app that lock my screen with pattern lock and use them simultaneously? Will it cause any kind of clashes or other troubles?"

That depends on the app, the lock-screen is within the framework of Android itself, unless the app over-rides the built-in lock-screen mechanism, then there should not be any clashes. The important keyword is over-ride. 

If the said alternative lock-screen clashes then that's down to poor programming effort. :)