Quote:
	
	
		| ...so they are [cancelling] each other out? | 
	
 Two signals on the same frequency and at nearly identical power levels at your antenna... yes, that very likely what's happening (though 'interfering with each other' is a more accurate term than 'cancelling').
As already mentioned, if you are up to the task of custom building a highly directional antenna, you might be able to pick one or the other CH-15 signals.