Onsite guests also get EMH from check in to check out. So 2 days of EMH (if they're on those days) with 1 night, as long as you check in early enough.
but with 4 parks, doing one nighters is hardly convinient. It's more efficient at Universal where you get 2 days with express for 2 parks
the next Disney one nighter benefit would be to avoid parking fees for both days.
You still pay $20 per night at universal to park at their hotels, or $18 per day at the parks even if you stay onsite at their hotels.
one night at a Disney hotel would grant 2 days of free parking at the disney parks and hotel.