Is buying these no longer an option? We have a 4 day trip planned for the beginning of April and were just invited to join family's 5 day trip at the beginning of May. I can't find them on the Disney website.
You can add the non-expiration partway through your first vacation.
Disney stopped selling non-expiring tickets via its website because so many guests mistakenly thought they needed non-expiration for tickets not being used immediately.
Note: Non-expiration might not save you money. For covering two vacations, non-expiration is a close call, not a definite buy. It needs to cover three vacations (of course in the 3 day length each) to really save you money.