If both husband and wife have each paid enough NI contributions to qualify for their full state pensions individually, they each get the full single pension.
If any one of them have not paid enough to qualify for a pension, or a very reduced pension, then they will get the married couple's pension.
So, if two x single pensions are greater than the married pension, they each get their own single pensions.
If two x singles is less than the married couple's pension, they get that instead.