You definitely DON’T want to book it as 2 different reservations if you’re going to buy the Disney Dining Plan with both resort stays, it would be unnecessarily expensive.
Remember, in order to book the DDP, you must do it as part of a package, and a package is defined as a bundled purchase of room + tickets. This means if you book a 3-day package and then a separate 7-day package and you want the DDP with both, you’d have to buy your tickets separately and that drives up the cost. In this scenario, the best you could do (assuming you’re going to a park each day) is book the first part with a 9-day ticket and the DDP and then book the second part with a 1-day ticket.
If you MUST book it as two separate stays …
My advice is book the first stay as a package with the DDP and a 10-day ticket, and then book the second part as a room-only and then pay out of pocket for your meals starting on the day after your second check-in (remember, DDP credits are good through 11:59 p.m. on your check-out day).
So here are some ticket cost scenarios:
A 10-day park hopper: $375
A 9-day park hopper + a 1-day base ticket: $454
A 3-day park hopper + a 7-day park hopper: $644
With those numbers, you can see why I’m advising what I’m advising. You can get 10 days of park admission for $375, or you could get the very same 10 days of park admission for $644. It all depends how you book it.