I think we are on the same track. The chance of there being another set of twins with the same names must be very high. If your great grandfather was James Alexander Addison then your great great grandfather was James Addison b.1833 and your great great great grandfather was William Addison b.1789. I have lots of information on the Addisons in the 19th century but the trail peters out at the start of the 20th century. Drop me an email - I am sure we have information to swap.