Boosting two accounts with a third, the two accounts will level faster then two accounts questing.
So for 1-60, option A will result in 60 (or 60's) faster then option B.

If you cascade granted levels.
IE link them A -- B -- C
For every 60 boosted on BC by A, A gets:
30 levels from B and 15 additional levels from C.
So you could level a toon to 15th on A, and be very close to 16th.
And then boost them to 60th, and ding 61st on exploration before arriving in Hellfire.

If they're equally geared, I'd boost with the Feral.

Once you are 60th, and are doing the 60-80 grind.
Three toons would probably be the booster + 2 new accounts coming up.
You lose the RAF experience however you look at it, at 60+

I would probably go..
1-10 Quest with booster in party.
10-20 Shadowfang Keep (Horde Option)
10-16 Deadmines (Alliance Option)
16-20 Stockades (Alliance Option)
20-45 Scarlet Monastery
45-55 Stratholme
55-60 Ramparts
60-66 Underbog
66-68 Black Morass (don't start event)

Having some kind of healer in each pair will help reduce downtime.
But you'll probably outgear everything so badly you won't need any.
You can move to Northrend at 68th.