I believe there is a setting for Proc rate.  Maybe you have to adjust that for the 2 heal spells you want to alternate.  I think if the proc rate is set to 0, it won't cast it at all.  But that field may only be for damage based spells.  I haven't played around with it that much, but I have noticed the same issue you are talking about before. 
		
	
		
		
		
		
		
		
			
		
		
		
		
		
		
	
		
		
	
	
	 |