Canoe Spacing is About Teamwork
Think of three things when deciding space between canoes in your group: communication, rescues and interference.
Stay within sight, be close enough to rescue quickly and don't be so close that a collision or interfering with another canoe is possible - especially in waves and in rapids.
Communicate or Become a Divorce Boat!
Communication is key to fun and safe tandem canoeing. But it's not easy! Both paddlers need to know the skills. A standardized vocabulary is best, for instance "sideslip to the right" if used consistenly is a very clear way of communicating where you want your canoe to go. Askiing your tandem partner to "pry" doesn't tell them what you want the canoe to do. It could be sideslip or spin. They'd probably like to know which!