Is it ok to mix full synthetic and synthetic blend motor oil?

Don't worry about being a quart low to drive to the store to get another quart, unless the store is several miles away on the freeway. I would rather run a little low for a short drive than mix oils especially at an oil change. I use synthetic oil for a reason and using petroleum based oil, even a blend, compromises the reason I use synthetic. But as far as hurting anything, no it will not hurt the engine or the oil to mix the two you mentioned. Mixing oils of the same brand is a fairly safe practice in a pinch but don't mix brands unless it really is an emergency.