@schueppert, thanks for your comments, and that does seem like a reasonable approach to buy two 7kWh units instead of a 10 and a 7 though it would have slightly less total backup capacity (14 instead of 17).
I am not convinced that Elon said the use case I posed was not currently supported. He was not completely clear on that, in my opinion.
I am not convinced that Elon said the use case I posed was not currently supported. He was not completely clear on that, in my opinion.