But i need both to work @ same time
Only one can work at a time. Internally, at the time of rendering both results in client side onclick event. Thus, one overrides other.
If you want execution written in both, then keep only one and append the operation in it. Like:
OnClientClick="if(confirm('Do u like to Continue?')) { document.getElementById('hdnTxt').value=1 } else { document.getElementById('hdnTxt').value=0;}; return ShowConfirmation();"