Higher waterpressure through one 40mm pipe or two 32mm pipes?
For a given volume of water per second, the two 32mm pipes will have a lower pressure loss than the single 40mm pipe.

Using the head loss calculator at http://www.engineeringtoolbox.com/hazen-williams-water-d_797.html, and assuming you're using PVC pipe, I found the following pressure losses. Each line gives the flow rate, the loss for a single 40mm pipe, and the loss for two 32mm pipes:


0 liters/second: 0.0 kPa and 0.0 kPa (obviously)
0.1 liters/second: 0.3 kPa and 0.2 kpa
0.2 liters/second: 1.0 kPa and 0.8 kPa
0.5 liters/second: 5.6 kPa and 4.6 kPa
1.0 liters/second: 20.1 kPa and 16.5 kPa
2.0 liters/second: 72.5 kPa and 59.4 kPa


And, just in case you're an avid XKCD reader, if you're running Niagara Falls through your pipes:


2831684.7 liters/second: 1.78 x 10^13 kPa and 1.46 x 10^13 kPa


(1.46 x 10^13 kPa is 40,000 times the pressure at the center of the Earth, or about the pressure at the center of the Sun. Moral of this story: don't try running Niagara Falls through your pipes.)