Two connected films are increasingly made as a single production stretched across one long schedule. The reasoning is logistical, and most of the saving comes from things that would otherwise have to be built twice.
Sets are expensive to build and cheap to keep
A standing set represents weeks of construction, dressing and lighting design, and once photography ends it is normally struck to release the stage for another production.
Rebuilding the same set later means paying for that work a second time, and matching it exactly is difficult even with the original plans and photographs.
Shooting both instalments before striking anything converts that duplicated cost into stage rental, which is far cheaper than construction.
Cast availability is the tightest constraint
Leading performers commit to projects years ahead, and reassembling the same ensemble for a second block often means waiting for the least available person.
A single continuous engagement removes that coordination problem entirely, and it is usually negotiated as one contract covering both films.
The same applies to key crew, since a cinematographer or production designer who moves on takes a great deal of undocumented knowledge with them.
Continuity of appearance cannot be recreated
Actors age, and where a story spans days rather than years the gap between two separate shoots becomes visible on screen in ways makeup can only partly address.
This weighs heaviest with young performers, whose appearance can change substantially across the interval a conventional sequel schedule would require.
Filming continuously means the physical continuity is real rather than constructed, which removes a whole class of problems from the edit.
Locations and permissions do not stay available
Practical locations change hands, get renovated or become unavailable, and permits negotiated for one shoot do not carry forward to another attempt years later.
Weather and season also have to be matched, and a location that reads correctly in one part of the year may be unusable in another.
Capturing everything in one visit avoids relying on a place remaining as it was, which is not a safe assumption over a multi-year gap.
The approach concentrates the risk
Committing to two films at once means spending on the second before any evidence exists about how audiences receive the first.
If the opening instalment disappoints, the follow-up is already largely shot, and the remaining choices concern how it is finished and released rather than whether it exists.
That is why the pattern appears mainly with established properties, where the audience is understood well enough to make the upfront commitment defensible.