I'm not sure what the format will be like, as far as whether it will be focused on Lindsey's solo work or will include Mac songs as well. Since they named him as the guest with Stevie as his special guest, it looks like they will focus on his solo work. That's just a guess. But why now, when he is on tour with FM and all about pushing SYW forward? Obviously, we know they will play "Landslide".
It seems to me it would be advantagous for the two of them to sing some FM songs together, perhaps from the new album as well ("Say Goodbye" comes to mind) since they are on tour to promote the new album. On the website they showed other "duos" that performed together, and they each sang solo songs and sang some together as well. (Trace Adkins w/ Travis Tritt and Peter Cetera w/Amy Grant are two mentioned) We'll have to wait for the show to know for sure.
Regardless of what the two of them sing or who the focus is on, it doesn't matter to me! This will be a very special concert. I can't wait to see it.