I think, quite simply, BS, because it is unfair. It seems they want to contribute to the bills in proportion to their earnings (and hence have spending money in proportion to their earnings).
Perhaps the best way to demonstrate the deficiency of your method is to imagine one of them got a huge rise. If the higher earner suddenly had an increase from £1964 to £10000 (and the bills remained the same) they would have an income of £11,531, bills of £2,000, so would share £9,531 (so get £4,765 – more than £3k more than earned!).