There are quite a few threads on split stays, but to get you started, I've written a primer on Disney World (that I give to family, friends, etc. on request) where I discuss split stays as follows:
"One way to take advantage of Disney discounts that do not perfectly line up your anticipated travel dates is to book two or more consecutive reservations. For example, suppose you plan to arrive at Disney World on September 4th, but a
free dining promotion does not start until September 6th. Aside from moving your entire trip back two days, you can simply book a two day reservation from September 4-5, followed by a separate reservation beginning on September 6th (in order to take advantage of free dining). The reservations can be at the same or different resorts.
In that regard, you can make any number of separate resort reservations for the same trip (with the caveat that each reservation will require its own separate deposit, based on the applicable room only or package reservation booking rules). Any trip that includes two or more consecutively booked reservations (whether at the same or different resorts) is referred to as a 'split stay.'
In addition to providing some degree of flexibility in enabling use of discounts, split stays can provide other advantages. For example, suppose you'd like to have a dining plan for part, but not all of your Disney World vacation. A split stay enables you to do this by booking one package that includes a dining plan and a separate package that does not (alternatively, you could book different dining plans with different packages).
Another key potential advantage of split stays is that they can enable more convenient overall access to the parks (and restaurants, etc.) than staying at a single resort. For example, a carefully planned vacation that includes a split stay with a first reservation at a resort near Magic Kingdom combined with a second reservation at a resort near Epcot and Hollywood Studios can enable relatively easy access to all of those parks (as well as restaurants in resorts near those parks). Some also like split stays because they allow experiencing two or more resorts on the same vacation.
Split stays do, however, involve some potential drawbacks. The most obvious drawback with split stays in separate resorts is that you must pack up and change resorts in the middle of your vacation. Disney, however, endeavors to make this easier by transporting your belongings to the next resort (via bell services) free of charge. Magical Express can also be arranged to take you from the airport to your first resort and then return you to the airport from your last resort.
Another potential downside relates to MYW tickets. As discussed above, the cost per day of MWY tickets decreases the more days that are on the ticket. Consequently, an 8 day MYW ticket is substantially less expensive than two 4 day tickets (or any combination of two tickets adding up to 8 days). Accordingly, it is financially inefficient to buy separate sets of MWY tickets for each resort stay and, instead, advisable to only buy one set sufficient for your entire stay at Disney World (preferably with either your first resort reservation or separate from any resort reservation).
Keep in mind, that if you want a dining plan at each resort, you must buy at least a one day MYW ticket for each person for each resort stay. In this situation, it is still advisable buy one set with your first resort reservation sufficient for your entire stay at Disney World, and then save any additional one day tickets (needed to book a dining plan) for another trip (unused MWY tickets never expire). Alternatively, if the lead guest on your reservation is an AP holder or you are renting DVC points, you can book ticketless passholder plus dining packages, thereby waiving the requirement for a one day MWY ticket for each party member at each resort."
