Subodh
In general it does not work well to configure backup on both ends of the leased line. One of the issues is that backup-interface puts the backup interface in a special state where it will not initiate a call and will not answer a call. So if you configure router B with a 10 second delay and configure router A with 15 second delay then when router B delay expires and it attempts to call router A will still be in the state where it will not answer calls.
You might get around this restriction by configuring dialer profiles and pointing the backup interface to the dialer interface rather than to the BRI. But if you do that you will get a call from B to A at 10 seconds and then get a call from A to B at 15 seconds. I am not aware of anything that will have A call B only if B has not called A.
In general I believe it is best practice to configure backup interface on only one end of the connection. You might also consider some alternative methods of establishing backup capabilities such as floating static routes or dialer watch.
HTH
Rick
HTH
Rick