I usually get it done in the same time, but save myself a lot of trouble and instance switching.
Using an 80 DK Frost tank, I level like so:
10 - 20 Shadowfang Keep
20 - 45 Scarlet Monastery
45 - 60 Stratholme
My DK has a Chopper, so I throw 1 alt in it and drive them to the next instance, then friend summon the other alt. The beauty of this route is that all of the instances are literally connected by the same road.