Think of the rear core as an extension of the front. There isn't an additional thermostat inline that would prohibit coolant circulation from front to rear.
The petcock at the passenger bottom of the radiator is lower than both cores, so as long as draining on a flat surface, maybe a little nose downhill grade, gravity will ensure a good of a drain as you are going to get.
I drained mine, refilled with distilled like you planned on doing, ran a few miles with front & rear heat on, and what I emptied contained very little coolant. There wasn't enough to make me consider a second flush. Had a few gallons of distilled on hand and using it was easier than dragging out the garden hose for tap water anyway.
The rear is integrated with the front via a Y hose with plastic (neoprene) connections that are somewhat leak prone. Its above the passenger valve cover area. It was this heater hose replacement that instigated the whole job. Its up to you if you want to proactively change this hose out now or not.
Only thing I'd do differently next time around is to attach a hose to the petcock and run the old coolant into a bucket. I used a very large catch pan but was still messy, especially on the frame/suspension. I also would NOT consider adding a garden hose connection, which is what most (ie: Prestone) cleaners recommend.
I'm not aware of any Expedition-specific videos, but there could be some out there.
Somewhere on this site somebody has the cooling capacity listed. The rear unit really doesn't add as much as you might think it would.
