Both involve having a very good rack with wide bars.
Put the canoe on upside down and full over to one side (best to use stoppers). Lash it down tight. Lean the kayak against it. Lash it down.
Second is to use an upright bar and lash the canoe to the uprights on one side and the kayak to the uprights on the other.
- Big D
Touring Kayak Paddles
Canoe/Kayak Storage Racks