If you are doing a split stay, you can book up to 10 days from the arrival date at your first resort, regardless of when you check into the second (or third) resort.
You can also book up to 10 days from the arrival date at any subseqent resort, once the 180 day mark for that resort hits.
For example, say you are doing a split stay with 7 nights at a first resort followed by 7 nights at a second resort. You can book for the first 10 days (including the first 3 days at the second resort) when the 180 day mark for your first resort hits. Then, when the 180 mark for the second resort hits, you can book all of the remaining days.
One thing to keep in mind is linking a dining plan with restaurants that require credit card pre-pay (e.g., CRT and dinner shows). Suppose in the above scenario, you want to eat at CRT on your 9th day. Ideally, you'd like to book this at the 180 day mark for your first resort reservation AND use a dining plan to avoid having to pre-pay. However, since this is after your first resort reservation is expired, you will not be allowed to use a dining plan at the 180 day mark of the first reservation to avoid pre-payment (even if you have a dining plan for both reservations).
Instead, to avoid pre-payment, you have to wait until the 180 day mark for the second reservation (note that you still can book from the 180 day mark of the first reservation but, in that case, you will have to prepay). Confused?