Doubt it as you can still be jointly assessed even if separated and living in different houses, it depends on circumstances and what the legal situation. Equally you could have had separate assessment while living together. Tax assessment is probably not much good for proving living separately unless it's clear from the way it's set up that maintenance etc is being paid.
We lived together till mid November sold house end November, divorced end of December...just wondering why the " date of separation" in these circumstances is required if not to adjust tax ? Presumably over last 4 years , as that is required time for a divorce whether living under same roof or not