The fow rate should act as though the pump is soley hooked up to the 1" diameter pipe, rather than the 2" diameter. Correctly, there will be turbulence at the junction, as well as more friction relative to volume moved with the smaller pipe. This is because more water is in contact with pipe wall relative to volume being moved. This means that the flow rate will be faster since you are reducing the cross sectional area, but the efficiency of flow will be greatly reduced by frictional forces. Anyone know the frictional coefficient of PVC (hehe)? This actually a quite simple problem if there are anyone on this thread that are good at calculus and physics. I'll post back later after talking with a physics friend from UNC-CH. I actually agree with the tapered couple idea to reduce turbulence (friction).
