I actually use Sanke fermenters, so I use a no-chill method to double-batch. Mash, boil, dump while boiling into the Sanke, cover with orange carboy cap with holes capped. I then make a new batch right away, or I've brewed on successive nights. I try to adjust my hopping to work with some of the general rules of thumb for hopping no-chill brews.
I've also done it where I brewed two batches of the same beer, chilled in-kettle, and combined in the fermenter. A couple times that's been a two-day affair, as it's been said, if you have good sanitation, a couple days is fine.I always pitch once I have the fermenter full, and at pitching temp, and oxygenated.
I've never tried the 5 gallon starter idea, though I might give it a go on something in the future. I think the advice given, to pitch full yeast amount, aerate big on the first 5gal, then add the second batch un-aerated as soon as it's at the current fermenter temp, might result in success.