Let's hope you can get it sorted locally and it works well. I suppose maybe a "y" piece after the rear box and a piece of pipe across to other side might work until you can maybe get a proper dual exit exhaust.
depends how quiet you wanted it? and as the tail pipes are quite short you would have a job to hide the y piece after the box.
some model Vectra C's came with twin exit systems so should be able to find one some where?