A second satellite could be put in the same orbit around the sun as the Earth, but on the other side of the sun. That would give us stereo vision for the parall

A second satellite could be put in the same orbit around the sun as the Earth, but on the other side of the sun. That would give us stereo vision for the parallax measurements. But it's cheaper to have just 1 satellite (which is also easier to communicate with because near Earth), and then compare images 6 months apart to get the same data.