I think you'd only be getting half of the benefits of a starter. A starter works well because the yeast multiplies (which using two packs would accomplish), and also because the yeast gets moving and is very active when it's pitched (which only a starter can accomplish.) But don't take my word for it.
My first couple of batches I kept avoiding a starter just because of laziness. I finally did one on my third batch, Its actually really easy and the results are amazing compared to pitching a wyeast XL smack pack directly. I won't brew without one now
6oz malt extract (liquid) in 32oz of water. Boil 10 mins, cool to 75f, aireate, add yeast, place concoction in 64oz bottle (sealed, no blowoff or ferment lock).
The last time I did this, the beer started fermenting just fine and the beer is now in secondary.
This has brought up a question however...
There is no 'foam ring' around the top of the starter as there is when beer is in the primary. What is this foam ring? Will it not appear on the starter since there are not solids from such things as hops or hot break (at least not as much hot break). Is this ring not forming since I have the starter in a sealed bottle unable to vent continuously and therefore does not bubble up (I do crack the lid every now and again to make sure its not pressurized)?
One sign I have looked for is the buildup of a sludge on the bottom of the container. This starter does indeed have a good buildup on the bottom. Is this truly a sign of the fermentation and yeast population? I imagine it is since this is what (or so I have read) you collect to use as yeast in future batches.
The first time I did this, I got a very slight foam on the top but this time, nothing. I guess I'm a little paranoid about this yeast I got. It was Wyeast 1272 Ale II liquid in a tube and when I went to pitch it, I found that the tube has a small pinhole leak in the end of it. I noticed this due to the tube being damp on the outside. I squeezed the tube gently and heard the air releasing from the pinhole.
Well, I'm off today and this is the starter I intend on using. Please reply soon if you read this and feel using this would be in error