[capink]

Right after the first action, insert a new "Chain Varibales" Action that defines a variable called do_continue with the following template:

program:
	start = format_date(globals(_previous_action_start_time), 'to_number');
	lm = format_date(last_modified(), 'to_number');
	if lm ># start then 'yes' else 'no' fi

After that set a condition on the last two actions as follows:

template = program: globals(do_continue)
datatype = text
comparison = "="
condition value = yes

Edit: The above assumes the first two actions are tied together. If there is a chance you are going to cancel first action but proceed with the second, you have to repeat the chain variables step after the second action.