We ride the tandem most evenings and on long, flattish rides.
I ride my titanium on hillier rides and longer rides.
My touring bicycle is new to the pack and has been used for some short evening rides and a ride on a rail trail ... a surface I wouldn't take the titanium on.
My Giant usually rides the trainer, but can be used on the road if the titanium is undergoing repairs.
My mountain bike will be used on the gravel roads around here, but it needs a bit of work.