That would not be a bussiness rule.
Business rules should be collected, and ideally be referred to in the source code.
They should describe the logic with respect to all business aspects: after printing an invoice, the invoice number cannot be changed anymore, and has to be put in the audit log.
Now one can have a business rule: send a notification on this ... event.
The implementing code may refer to this business rule, and state that if needed it splits a too large message in two parts separately sent.
That does not look like a requirement on beforehand, but an implementation documentation afterwards. A bit like "one can click on the i to receive contextual online-help."
If one only has business rules, fine, add the implementation details. Otherwise keep them separate, keep the business rules' ownership neutral and without too much fluff: the business people should not feel overrestricted by imposed details/"corrections." It is like pagination of lists and such. "The results must come in limited pages, though an entire scrollable list must be selectable too." That is - as you said - something for application design. And - in contrast to decisive business rules - tells something on the interna.
Having two messages must be explained. But let me give a comparison:
An architect tells: for a building for N people and M floors there are K lifts. The building owner will want to have the technical documentations: how the elevators intelligent wait on first and top floor, what strategy to respond to a button press and such. Important technical implementation details, intelligent design decisions. Sending two notification messages falls in the same category.
In the business rule "notification message" needs to be changed in "notification as one or two messages (foot note: if the notification becomes too long)" but technical justification and details should go elsewhere.
Now the implementation may be changed without the business rules being effected much (those rules will deal with a "notification", not a single partial "message").