@Cindy527
The host has no control about the way the guest is paying Airbnb. The "split" feature does excists, but is offered by Airbnb to guests under certain conditions, which are not under control of a host.Please note a reservation more then 28 nights is a "long term reservation" with the "long term cancellation"policy automatically applied.
You could offer the guest half a month by special offer,block the remaining dates, and later change (extend) the reservation to the check out date. Then the payment is also split in 2 installments. But be aware of the cancellation policy not being "long term"!
Also the AIrbnb Guest Service Fee percentage for short stay is higher then for long term stay.
Best regards,
Emiel