According to spaceflight101:
[The Soyuz capsule] is outfitted with the spacecraft's fully redundant parachute system [...]
I would assume that the designers needed to keep a certain precision for the landing. If they deployed the main parachute and the backup simultaneously, the decent speed may be lowered to the point that they could no longer satisfy their landing area requirement, because a slower decent always means that there is a larger potential for the spacecraft to be moved by winds.
According to Popov's "Surprises in orbit" the backup parachute is smaller, with only 574m² (instead of 1000m²) area, leading to an impact speed of a whooping 8-11 m/s, which is far more than is comfortable. (Thank you, osgx)