>> ... we haven't tried to each read the same book at the same time ...
Make sure a fire extinguisher is in the room when you try that.
>> ... Do many people here actually buy two copies of an ebook, when they are to be read in the same household?
"Read in the same household" isn't a phrase I've run across as an excuse to duplicate purchases as I peruse the legal agreements that I have entered when dealing with book vendors. You know, those "license agreements" that no one seems to read as they log on to Web sites.
BUT THAT'S NOT WHY I STARTED THIS THREAD! I just found it amusing that Kobo beat Amazon, in this instance ... (grin)