Some time ago I read a book about 2 men meeting at a road blocked by a fallen tree on a french road. As they could not pass they agreed to exchange places the one heading to london returning there as the other person and visa versa. I think it was set between the Wars or just after WW II
I know the book you are looking for ....will rack my brain cell..... was it something like my brother's keeper ?? I will get back toyou...I know I read it in a Reader's digest format years ago.
And I donate my lunch hour to you !! I found it by wiki and scrolling tho until I recognised titles then reading the plot ( or even loosing the plot !!) must get back to work. I enjoyed the trip down memory lane. PS Amazon has it but also try abebooks.