You really don't need two kettles to brew all-grain. I used a single, 8-gallon stainless kettle for three years. You can use it to heat your strike, then sparge water, and when it comes times to sparge just collect the runnings into a bottling bucket. After you've collected your pre-boil volume, just transfer the wort back to the kettle. Voila.
By the way, I now have an aluminum kettle for strike, sparge water, so you're well ahead of me.