In the real world, a relationship like this might exist...plural marriage, open relationships, etc.
However, history and public records reveal plural relationships are not always as sweet and succinct as this combined commitment of the (fictional) Kate to David to Jane.
Based on the premise of this story, I do NOT think this kind of a relationship could survive between the three characters, especially considering the "strength" of the two women. Lost my sympathy with the hubby, David when he tried to have both women on a level playing field.