Don't bother - SSDs get faster as they grow in capacity. Why take the double chance of losing all your data? (Especially with the 840s, which aren't nearly as reliable as 840 pros.)
As far as I'm aware the 840 is just as reliable as the 840 Pro. It's less durable, but that's something entirely different.
As for the question, I'd recommend a 250 GB Samsung 840 over two 120 GB Samsung 840s. It's still faster, though not twice as fast, but it's significantly cheaper per GB.