Take 2 coaches. 1 x TGS and 1 x FO.
Cut the guards area out of the tgs including the 1 passenger window, cutting as close to the next frame as possible
Cut the end two windows out of the one side of the FO leaving as much frame as possible
Insert section of TGS into FO side and trim guards end to fit.
File, sand, paint and glaze
Should about do it
Thats assuming the other side is just windows as per a normal FO?