This is an anti-pattern because you can't model "1:1" that way. Instead, it will be "1:0..1", and now you have some nasty corner cases when the first table has an entry whose counterpart in the second table is missing. Also, when using a column now you always have to think about which table had it - the first or the second table?
I had to work with such a design in a real-world project and it was really annoying.