The dose is first doubled and diluted to a total of 10ml. Doubling the dose helps to increase the effective amount absorbed, since the lungs don't absorb all of the drug very well.
Some think that you can double an epi dose by giving 20cc of drug down the tube (2 x 10cc of 10:000), but that is a lot of fluid for the lungs to absorb (even the 10cc amount can be a bit of a challenge).
So how does one manage to get twice the amount in 10cc? You have to use multi-dose vials of the drug (higher concentration) and dilute with saline to 10cc (total). Sounds good in ACLS class, but I've yet to really see it work efficiently or effectively in or out of the hospital. Not to say it doesn't, but I wouldn't put much faith in the endotracheal route, especially if the pt is/was in CHF or has aspirated and already has fluid filled airways.