Hmm, I think you have to mine few blocks before difficulty adjust. How long does it take mine next block on 2 laptops ? (but I'm not sure)
Yes, difficulty adjusts every 2016 blocks (two weeks on average).
At current difficulty mining a block with two laptops would take, pretty literaly, forever.
Thats not really an issue though, unless hashrate drop incredibly fast during a single cycle it wouldnt matter. Assuming those two laptops have at least the mininum hashing power for difficulty level 1 (roughly 7MH/s), they would mine just as fast as the whole network currently.