Is it normal for the two channels frequencies to interfere each other?
Yes it is called cross talk, you can minimise it but you can never get rid of it all. Two circuits running so close to each other are bound to give you problems. One way to minimise it is to have good supply decoupling, using inductors in a Pi filter like I show for a motor on this page.
http://www.thebox.myzen.co.uk/Tutorial/De-coupling.html