add this to button or etc.
xxxxxxxxxx
data-dismiss="modal" data-toggle="modal" data-target="#Second_modal"
$(document).on("click",".signUp",function(){
$('#id_of_sign_in_modal').modal('hide');
$('#id_of_sign_up_modal').modal('show');
});
adding $("body").addClass("modal-open") solve the scroll problem as shown on the sample code