Which sounds like a terrible trade-off, harm isn't linear here. Two leaks, each of 1000 barrels, will badly damage two different valleys. One leak of 2000 barrels would be better.
And cost isn't linear either, two half-capacity pipes will probably be almost twice the price. You could spend that on more steel for the large pipe instead.
Er... I don't think you've completely taken into account what you're proposing. Due to the square-cube law, splitting a pipeline into two smaller pipelines means that in order to have the same total cross-sectional area you would need sqrt(2) times as much surface area. Not only is the surface area that is subject to corrosion increased, I would imagine the pipe wall thickness would probably decrease so corrosion would compromise the integrity of the pipeline even sooner. It's also double the number of welds that need to be made, double the length of pipeline to maintain, and as to the notion that it would do half as much damage, I doubt this claim. Why would it actually substantially decrease the damage to the environment? It's not like pipeline leaks completely sever the pipeline in half and just start inundating the surrounding area with a flood of oil. It's usually a crack or pinhole leak, so the smaller cross-sectional area of the pipe probably won't do much good in terms of decreasing the volume of oil leaking out.
Actually, I'm not so sure about doubling the risk. Failures may appear random, but they aren't. A failure may indicate a particular defect that can be corrected in the rest of the system before another one fails.
And cost isn't linear either, two half-capacity pipes will probably be almost twice the price. You could spend that on more steel for the large pipe instead.