It has been tried by a fair number for runners
There are succesful 400/800 runners but not that many
The 44.5 - 45.5 runner would have a massive incentive to step up to 800m and be among the best at 800m if it was so easy. There is a reason that most don't try it!
Another reason is that a lot of 400 runners are 100/200 runners stepping up and consider the 400 to be pushing their limit