Personally, I am gonna have to say, I would NEVER do this. Sure tuck some into the existing landscaping. But to plant a whole yard in two plants? May sound nice, but i assure you, it wont look it.
Cause in the winter your yard is gonna look like something the Adams family would plant.
Chamomile is a reseeding annual. It is also very tender. It will die back and be nothing in the fall and winter.
From what you have described, I have a mental image that you are trying to make a wild flower looking field type thing in your yard. Is this what you are going for???
Lavender is not pretty in the winter and I am not sure how well it over winters in your area or how well it does. I have to be pretty careful because my area is too humid for most lavender types.
Please explain to me what you are going for in terms of what you want, how big your yard is, is it close to the street, and WHY you would want to do this.