The core decision

Two campuses or two departments?
The question is about identity, not size.

The decision is not primarily about the number of students, subjects, or teachers. It is about identity: do the two programmes need to present themselves as separate schools to their respective audiences, or are they branches of the same school? A secondary school and its sixth form that share a name, share teachers, and present themselves as one institution should be departments within one campus. A secondary school and an independent evening tutoring programme with separate branding and separate student bases should be separate campuses.

Three questions that determine the right answer

For your
specific situation.

1️⃣Do the two programmes share a brand identity?
Same name, same logo, same public presence → departments. Different branding, different public profile → separate campuses.
2️⃣Do the two programmes share students?
Students enrolled in both simultaneously → probably one campus with two classrooms. Completely separate student populations → separate campuses make administrative sense.
3️⃣Do you need separate marketplace identities?
Two creator profiles with different branding → separate campuses. Happy for all curriculum to be associated with one school identity → single campus is simpler.
When in doubt

Start with departments.
Add a second campus when you are certain.

Adding a second campus is straightforward and can be done any time. Reorganising two campuses into one is significantly more work. Start with a single campus and departments unless you are certain from day one that you need two separate campus identities. If you build with departments and later discover you need two campuses, you can add the second campus and migrate relevant classrooms in under a day.